I don't know what to do

Me and my boyfriend meet awhile ago and it's both of our first official relationships with someone. Since I'm inexperienced with relationships, I don't know what it normal. The problem I'm having is that we keep running into a barrier. He talks about sometimes how I could do better than him and what not, which I try to reassure him differently because I have the same fear sometimes. I try to talk to him about why he feels that why and how we can fix it but it never works. He doesn't know why he does that stuff he does that makes him think I'm should be with someone better. And when I try to tell him to think about it or fix it, he says that it's too overwhelming to do it and he doesn't want to change. He would rather be miserable. I don't know if I'm the one asking for too much or something. I just need help because I can't watch him do this to himself for much longer