I've had 2 of those moments in my life.
High school/college boyfriend who I realised I was only with because I was afraid that I'd never find anyone else who'd love me, even though we were absolutely wrong for each other and I felt completely suffocated in the relationship. I was going to uni, wanted to travel, work abroad maybe, build a career. He started working at 18 and wanted to settle down and have kids asap and kept pressuring me to stop my education and get a job.
The moment it clicked was when I told him I was having doubts about us (5 years in the relationship) and he threatened to commit suicide if I left him. I asked him: so you'd rather have me stay with you, knowing that I'm miserable, than give us both a chance to be happy with someone else? He said yes. We broke up a week later.
Then the first guy I moved in with. Spent 6 years trying to be so incredibly loving and nice to him that maybe one day he'd start being nice to me too. That never happened. My self esteem was pretty low and he made sure it stayed that way. Then one day I went off hormonal birth control and got a coil, started losing a lot of weight, mood lifted a lot, started feeling more confident. He went nuts, absolutely hated it and did everything he could to make me feel small again. I realised that he'd never care about me at all, that he didn't want me to be happy and he'd be the biggest obstacle on my path to happiness. I decided to become the person I wanted to be and stop caring about how he felt about it. He spent the next 6 months using all of his tactics to get me back under his thumb, but they all backfired. It was like I could finally see right through him.
At one point he started openly cheating with a co-worker, hoping I'd beg him not to leave me. I just shrugged and suggested we have an open relationship cause we were still young and should enjoy opportunities when they arise. He didn't know what hit him when I started going out on dates as well.
A few weeks after that he said that he was having doubts, because he thought he could do better than me. So I calmly said: okay, sure. Are you moving out or am I? He had a big tantrum after that.
The final straw was when he suddenly wanted to take me on a date (which was suspicious already), but just wanted to rattle me by leaving me alone on the date to hook up with another girl. So I just called a friend, went out with him instead and had a great time. He was expecting missed calls and me begging to come pick me up. He got really angry and became violent, so that's when I dumped him on the spot.