Why can’t I come?

I’m in a relationship with my boyfriend for 2 years now and we’ve been having sex so often and he is my first. I really enjoy having sex and everything and I get all wet and I think that everything is perfect but there is one problem, I’ve never come and he does every time. So I don’t know what’s the problem, why can’t I do it? I really want to try that.. When we have sex I feel really good and it’s awesome but I never get ‘’that feeling’’ and when he fingers me and finds my spot I always get that feeling and it is a beautiful feeling but than I get nervous and get another feeling that I can’t stand and push him away and that’s it. I can’t do it! Why? Does anyone know whats the problem? Am I not relaxed enough? What shoud I think when I have sex?