Can someone explain the on/off cycle

I’m with her and I want to break up, we break up and I miss her like crazy. She lives in a different city.

We’ve been 2 months NC and when I think about her I still feel very sad and like I miss her. I know going back with her is a mistake and I’ll end up wanting to break up again so I don’t want to perpetuate the cycle. I’ve been on the other side of this dynamic and it sucks.

I just want to stop feeling sad.