19 years (we share that, Khairy) of incompatibility. Me as a young man, an idiot who kind of fell into being married (I think you should have to pass tests to get married AND to have a kid before you're 35 - hehe). We lived together awhile, even bought a house (oh, as an investment) before we got married. I was young (23), unfocused, didn't have the slightest idea what I wanted to do with my life, but I was a hard worker and provided well. Hey, I had regular sex and we got along alright (at the time). I really did think I was in love, but looking back over time (which provides its own filters, of course) I think it was the lust and my particular ignorance of youth. I really think I had no idea what love was - and neither did she.
We really didn't have any problems the first 3-4 years.
But she hated my folks. That should have told me right there that I was marrying the wrong person. My folks aren't saints, but I have no doubt that my mother and father sacrificed an awful lot of their lives to give us a good home (not just a place to live, clothes to wear and food to eat) and did everything they could to ensure my brother and I started off life better than they did. My folks are good, upstanding people.
My folks tolerated her, for all 19+ years that they had to know her. But they never liked her. They treated her well, even when she treated them with thinly-disguised contempt. I should have paid more attention to my folks' feelings about her - having had kids of my own and now raising another batch and they coming into the same age where choosing partners starts to happen, I realize my biggest mistake then was not paying attention to why my folks didn't like her (and realize this is the arrogance of youth [disguising the necessity of experience!] that the young will rarely listen to their parents, and having seen a couple of the guys one of my sisters-in-law has gone out with, I know it's hopeless to hope for any different outcome related to warnings shouted from the rooftops).
My ex turned out to be a different person than I thought she was, and she may well have felt that way about me, but I think she knew me well enough and figured she could change me. We didn't communicate very well. I was simple, I really didn't get pissed off by very much, but she could sure do it to me. And it mostly started with what nlarrucia already said: "If you don't know or can't figure out why I'm mad then you are showing just how big a problem you have."
I stayed with my ex so long because my parents taught me that giving your word, and particularly as an oath in matrimony ('till death do we part), means something important in this world. She hated my folks, but it was their teachings to me that kept us together so long. In fact, I so didn't want to disappoint my folks that the last 5 years that my ex and I were together was solely due to that - I'd already figured out I was miserable and had to do something but couldn't disappoint my folks. Turned out I was quite an idiot, for more than one reason.
I'll never forget the night I called my folks (they lived some 900 miles away by then) to tell them that I was getting a divorce. I'd steeled myself to take criticism, to have to explain things, to withstand their attempts to talk me out of it, etc. After I gave them the news, there was a moment of silence and I thought "oh crap." Then my mom says "well, all I can say is it's about time!".
Then I come down here and meet a true angel from Paraguay. She loves her family (something my ex didn't - my ex's mother and father were both alcoholics and obviously that affected her, but my wife's parents have their own issues and she still loves them!), desperately wants to meet mine (we can't get her a visa without going the immigration route and my mother would have issues being on a plane for so long to get down here - we're thinking about meeting somewhere in the Caribbean some day soon - not such a long flight for mom
). She works not because she has to, but because she wants to contribute (she bought me a beautiful leather coat two years ago for my 50th birthday that cost her a month's wages!). We're not jealous of each other - our happiness isn't tied to us being together all the time. We have our own lives and we have our lives together. We have three "daughters" (her [much] younger sisters) who are quite demanding and we both share in their raising and in giving them a loving home in which to get a better start than their parents had.
My advice to anyone out there in a relationship in which they are not happy: Get the hell out of it as soon as you can. Life is too short. Be happy, not a martyr.
If you have kids, don't let that stop you. The idea that you have to be together until the kids are grown up is fallible: the kids will suffer more from the constant stress and probable fighting over the years than they will sharing parents while they grow up - and how you handle that part of it (post divorce) should be a better example for them than staying together and having to listen to the crap that comes out of a bad marriage.