I'm sure I've had many moments like that. A teacher in high school telling me that I am not dumb; maybe I wasn't great at math and I was only decent at science, but I was actually really good at writing essays. A professor in college telling me that there was "poetry in my prose" and urging me to write outside of my comfort zones while encouraging me to begin writing creatively again at all (I really miss her, I wish she was still alive). The moment I decided to send my best friend a goodbye note after attempting, lol, maybe I wouldn't be here if she didn't eventually build up enough bravery to intervene. In hindsight I can appreciate her for that. It's only given me more time to spend with her and more time to get to know her. She is truly a wonderful person.
Nowadays, I think my biggest defining moments have to do with my fiancé. What would have happened if I declined his invitation to meet? Where would I be if I said no to the proposal? I'm not happy right now, and the circumstances that I'm in are a big reason why I'm so suicidal. But where in the world would I be? Maybe I would be worse off, because I wouldn't have had certain experiences, I wouldn't have had a chance to discover more of myself and become who I am right now. But it's not like relationships are the end of the world, I would have probably found a different me otherwise. One that finished her degree and stayed in the same place that poisoned her, maybe. Or, maybe, I would have recovered and found a better life. But I'll never know, because I chose this path. I can only know whatever comes of the future in this specific route. And maybe it will be better, but if not, at least I know what to do.
I'm really sorry you had to see what life was like with your mom instead of your step-dad. Courts and life are fucked like that; children are property, so they go to who owns them the most instead of who can take care of them better. I'm sure you might have suffered a lot as a result of not being able to choose for yourself in that defining moment, especially since your mom is denying that you've ever voiced your own opinion. It's kind of comforting to think that maybe we would have had better lives if things were different, but sadly, they just are the way they are.