I was spurred to think about my infamous ex-girlfriend recently. Infamous, I say, amongst those who know me, because she serves as a convenient reference point for proving that anyone else is a wonderful person. But I did learn a few things from the experience, and it might be worth sharing them here.  I won't be exaggerating, nor angling for sympathy... just reciting what I've learned.

K wanted me to be bent around her little finger, and would have large tantrums (day-long tantrums were common; I once timed 5 hours solid of crying, closely followed by 3 more) if she didn't get her way. I was not to talk to other women, or my friends, but be devoted to her, to the exclusion of all other cares. I wasn't to have hobbies that made demands on my time, nor worry about my degree.

I saw a recent news story by a woman who had escaped an abusive relationship. She described how the man had fallen in love with her, been very charming, but had gradually become abusive. She described how the man induced an emotional dependence on him, so that when he quit his job and wanted to move away from her friends and social network, she did because she was naive and would do anything for true love. Another life ruined by the 'true love' myth, unlike all those lives utterly unruined by pornography or action films, men's equivalent enjoyment.
She was isolated, and gradually his behaviour became worse. He hit her, and was enormously contrite afterwards. She forgave him because she was sorry for him and his emotional issues. And it happened again and again, and she accepted it because she was kind and loving and wanted to help him. The abusive relationship was not based on physical fear, but excessive generosity and a desire to cure him of his own demons.

I sympathised. K, my evil ex-girlfriend, was similar. She liked to try to manipulate me by being hurtful. This might take the form of insults (she's the only girlfriend I've had who openly thought I was ugly), or the ever-so-subtle hints that she was desirable and could find attention elsewhere. The idea, presumably was to draw attention to how lucky I was and to my devotion to her, and spur me into whatever action she required. I doubt that I'd be gullible enough to be spurred into action even if I did love someone doing that; I have a stubborn streak that will outlast granite. But here we meet lesson number 1:
Every single time you tell your partner something hurtful, whether it's deliberately trying to invoke jealousy, being directly insulting, threatening to leave or simply asking directly 'don't you love me [enough to do that]?' you are actually telling your partner 'caring for me will cause you pain'. By setting up conflicts between your partner's love (if any) and desire for you, and your partner's character or other wants, you are stunting the growth of any love.
By teaching your partner this lesson, you ensure that you will get only stupid, masochistic or uncaring partners.

K also had an impressive ability to ask questions such as "are we ok?" I find this a very strange question to ask. Presumably she knew if she was alright or not and only lacked my opinion out of the two of us, but she didn't ask "Are you ok [with things]?" She said "we", which is the clue we need to work out what she really cared about. It wasn't me, and my feelings, but Us: the Relationship.
I rapidly took to capitalising it in a not-so-subtle display of displeasure. I had a strange idea that I would be valued as an individual in a relationship, not as merely a means to create the ideal state of being, which is The Relationship.
I have over the years had other girlfriends, and I have had arguments with them from time to time about previous girlfriends. With the notable exception of the evil K I am very fond of my ex-girlfriends, and pretty much all my 'flings'. If they come up in conversation I speak of them warmly. This has pained the odd woman, who would prefer me to dislike them. The unreservedly insane K went from being utterly in love to trying to tear out my hair and then sending me hate-mail.
I have been told and read that the desire for me to be on bad terms with them is so that they'll be less of a 'threat'. Clearly someone I've been out with I have some modicum of attraction to, and my current woman's status as my one-and-only is threatened. But if I hate all the ex-girlfriends, there's no threat. This only makes sense as part of rather warped attitude to relationships.
If you truly love someone, that love is not conditional. You want the best for that person: you wish them joy and happiness, and there's probably some degree of selfishness mixed in there, so that you want that joy and happiness to happen with you. Two millenia ago when Christianity was being invented people knew that unconditional love was the greatest and most precious. God loves us unconditionally; this is so central to the religion that it was the name of the campaign the Christian Union had to convert people when I was an undergraduate.
If you love someone, you won't impose the condition that that person loves you back, or spends time only with you, and you won't impose the condition that the person has to be in a relationship with you for you to love them. If, when a relationship ends, you find yourself hating your ex-partner then you didn't love that person properly in the first place.
There are plenty of people out there whose selfishness is always greater than their love. They need to stifle the object of their love: to hoard it and obsess about it. They care about themselves, or else The Relationship, a sort of Trojan Horse for this invidious attitude. By making everything about The Relationship they conceal the obvious selfishness, because both people should be doing things for The Relationship. There is no other person called Relationship. There are only people, who deserve not to be lied to.
With the contemptible K, I never told her I loved her. In fact, I specifically denied it on many occasions when she pressed me on the subject. I am, apparently, a cold-hearted ba**#@/] who is incapable of loving someone. I did admit to caring about her, although I never confessed the thought running through my mind that I care about humanity as a whole, and that one can care that another's welfare is bad. I couldn't be bothered with a week-long tantrum. She of course swore her undying love for me, a love that evaporated like dry ice in a volcano when we broke up.
I am tempted now to break up with any girlfriend just to check that she's a decent person who cares about me to some extent.
Lesson 2: there is no Relationship, a thing to be perfected beyond the two of you. There is you, and your partner.
It's normal to want to share the good times with your loved ones, but if you can't imagine liking your partner after hearing, upfront and with no cheating, that your partner is falling out of love with you (and in love with someone else), then you need to think about why you're with that person, and wasting the person's time.
I am annoyed that the common opinion is that people who do not fall in love easily are derided as malfunctioning dead-ends who cheat and waste your time, but the people who do not even understand love, and act on jealousy and selfishness before turning into raging animals, are simply naive innocents, hurt once again by the cruel world.

Honesty.
Whom do you find more creepy: the person who admits on the first date that he's into whips and chains, or the person who goes out on a load of dates with someone and begins to dream of marriage and children?
Again, the immediate cultural connotations are that someone kinky is the nasty person.
But I struggle to understand why honesty is so unpleasant. Not everyone's interested in marriage, and children are definitely a huge and life-changing goal. It's grossly unfair to get far into a serious relationship without sharing these life goals.
With the reprehensible K, I was always careful to be honest. Often that would be a tangled web of subtly avoiding answering the question in order to avoid missing dinner, sleep and breakfast, but frequently I admitted outright that I did not love her (and I rapidly came to despise her, and eventually I gave up on even affording her the general care that I believed I owed to all humanity. She was quite inhuman).
Every time she heard the truth about our relationship, she'd have an angry weepy fit. I learned a great many tricks for avoiding conversations and distracting her (and some not so subtle ways of ignoring dodgy subjects).
But even when I managed to avoid the hissy-fits, I didn't enjoy the suspense of never knowing if conflict was going to arise. I did quite like the puzzle of phrasing things carefully, but there are more fun puzzles to attempt.
Lesson 3: By punishing me for being myself, K did not change reality. She simply ensured that I didn't want to share it with her. If you are ever tempted to become angry with someone when they tell you the truth, think hard about how much you value both the truth and sharing your life with that person.

Cowardice
When our relationship was ending, and in many other relationships besides, people have called it cowardly to stay with someone rather than outright tell them that they're not wanted.
Cowardly to stay with someone? Or just damage limitation and kindness? She needed to take some responsibility for herself. If you get upset at truth, don't expect it.
On a similar note, we're always told that it's cowardly to break up with someone from a distance, in an impersonal way. Text messaging is worst, e-mail poor, and face-to-face best. Impersonal is bad, and it's cowardly to do otherwise. But by invoking the word 'cowardly', these people already tell us what's really behind this. There is something to fear, and it's the outsized response.
For every cowardly man who breaks up with a woman from a distance, there's a spiteful, vengeful monster who wants to be able to punish him for being honest. If a man doesn't trust a woman to behave like an adult, it's best that the relationship ends, and the woman should think carefully about her own behaviour before deriding his. This is an important one, but it's just a sub-set of Lesson 3.