Sexually shy?

My boyfriend is 100% comfortable with his body and he always says nothings wrong with me I’m perfect. But I don’t like my body so I never take my clothes off in front of him because I feel fat, my stomach isn’t as flat as it was before I got on birth control now that I’m off it I thought I would loose the weight But when we sleep together he wants to try all these positions and I can’t my anxiety makes me freeze. I love him and he doesn’t push me into anything it always agree to it. But he’s more sexually experienced like he’s watched more porn and has done more than I have I just feel scared. The other day we were doing it and I felt like I had to pee and I got scared but then it went away and I was completely calm but felt something gush out of me which I prayed he didn’t notice and I still haven’t told him about it because he’s so happy and excited and I’m sitting over here embarrassed. I also have general anxiety disorder so that doesn’t help. What do I do?