Being in your very intimate relationship(usually the first), no matter how many times you see the signs of toxicity and realizing this relationship isn't something your partner chersis. Like let me give you my example. The first relationship I was in what I realized very late or maybe I refused to realize was that I was a mere object to her to show to her ex what I was, that's all I was.
I didn't want to believe it at the moment, or maybe I wanted to but I expected things to work out somehow. I was treated like shit, but I kept my sanity for as long as I could and then the hurt inside me broke up. It realized that I can't let her destroy what little part of me there is left and when that hurt spoke up, suddenly I was the crazy one.
Anyways, I did try to break it up earlier, but she started crying in a very small restaurant and up until that point I was fine but when that happened I was like, "WTF happened here!".
I won't go much into that side more but what I did think in the later years is if there was a time machine and I could go back in time to tell myself anything, I felt there was nothing I could say that would make me end my relationship with her. Now I however do.
If I went back to myself, I would say to myself and this is the why you're looking for,
"She will move on without a single thought about you, without realizing that she has scarred you for life completely but you will not be able to if you spend even a little more thought for her. You know the signs, you have seen what she's like, how she's trying to manipulate you into thinking everything is your fault. No matter what you do, she seems never to change her attitude towards you. You have to save yourself for someone else. The damage she is going to cause will set back your life by decades if you ever manage to patch things up but if you end this right now, you would have saved yourself yourself for someone else."
That would make me end that in-my-mind beautiful relationship immediately.