Advice needed

I have been dating my boyfriend for about 6 months now, and we are great together. We only have decent sex though, and I feel the problem is that he has a slightly smaller penis then I am used to and he has a larger stomac then I have had before as well. I get very self conscious about the situation since I have trouble figuring out good positions. I know he enjoys himself but I have some trouble getting full enjoyment out of it sometimes.