What happened??

So I’ve been dating my boyfriend for over a year and a half now, and at the beginning, I was fine. Now in our relationship, I feel insecure about my small breast. I guess I’m afraid he’s going to leave me for a girl with bigger breast. I’m a size A and it just sucks knowing he can easily go find another girl. But he says he doesn’t want to, he loves me for me.. but I can’t help but feel insecure. He hasn’t gave me a reason to feel insecure, I just am all of a sudden. I hate it because I have a hard time watching a movie with him cause a girl in a bathing suit might be in it or something. When that happens, I just put my head down and tear up every time… this depresses me so bad