Changing Beliefs in Others/The way the world works, etc.

I have a belief that love is conditional, specifically with regard to appearance. I am scared that if I am in a relationship and gain weight, he will leave me or cheat on me. I’ve noticed that one reason I sabotage my weight loss is because I feel better meeting someone when I’m 10-15 pounds heavier than I’d like, just to have some wiggle room. I do believe I’m capable of losing the weight, but its this external belief that prevents me from fully committing. I also think it keeps me pretty guarded in dating. This belief comes from me seeing my father leave my mother in large part because she gained weight, and also seeing multiple friends have weight be an issue in a relationship or a cause of breakup. I’ve also seen most relationships, as well as my own, end in adultery. For many of those situations, the cause was appearance. I don’t know how to change the belief about this, the evidence seems to so strongly support it. I guess maybe I can change it to, “it’s ok to be alone,” but that doesn’t seem much more positive.