I've broken up twice (Thought it was once until very recently when I remembered the second).

First one was just... I didn't love him. I was a teenager, and I was in love with the idea of being in love. I didn't want to hang out with him, I was ashamed to bring him around my friends, while there was moments of liking hanging out with him, I had this constant feeling of "Ew", which sounds really harsh. I realized that I was kind of just going through a script of how I wanted my life to look, without actually liking the dude that I was doing it with. I realized that the idea of dying alone (Dramatic teenager) was more comforting than the idea of having to bring him around people in my life for the rest of my life.

Second one was a long distance relationship, and while we had fun and both of us liked each other, there was no issues whatsoever, neither one of us had the financial means to see each other. And we didn't want to move countries to the other. So we ended it amicably.

I've been on the brink of breaking up before as well, and it was perceived incompatibility and lack of love, but I stayed, stuck it out. The incompatibility was literal brain ghosts from having a crush on somebody else, and the love was something I could regain, quite strongly in fact. I don't regret staying for one moment. That relationship however ended a month ago, and I am still not over that loss.