I can't enjoy sex with my new boyfriend because of my ex!!!

so I recently broke up with a boyfriend of 4 moths and decided to date a guy I have known for quiet a while. He is really sweet and has always made it known that he likes me and cares for me. I like him too but the issue is I cant seem to enjoy sex with him at all. I sort of had sex with him a while back before dating my now ex and it wasn't all bad. However, when I got with my ex, everything was different. He was bigger( thicker) and more passionate yet aggressive too! Having sex with him hurt most of the times but with time I got used to that- I enjoyed the pain and got off of the pain of it all.

Well long story short, now am with my loving boyfriend and its just not working. I never really care about size , besides he is average or a little below- the problem is he cant stretch me out or even if he is aggressive it just don't hurt either way cause the girth is pretty thin and I cant seem to enjoy the sex no matter how much I try. Am I ever gonna be able to enjoy sex with him if he isn't big enough? I keep having sex with him though for his pleasure of course because I feel really bad and I don't wanna hurt him or his feelings but am afraid with time I will start to hate him cause and eventually lose interest!!!I tried focusing on other things like anal and oral but as much as I like giving that, I don't enjoy it. what should I do? is the situation helpless?