Kind of an embarassing question about anal sex

Hi everyone, I'd have a question about sex. 5 months ago I got the very first boyfriend, and we didn't wait so much to do some sex. At the beginning I was always nervous to do it, but now it's everything fine. I'm usually a top, because of... well, biological reasons (?) and he's bottom, because his ex boyfriends never let him be top. Sometimes I let him top because I see he does really want to do it, but I never reached an anal orgasm (while when I am top, it happends very often). I can't still find bottoming pleasant, because it hurts, so reaching an orgasm seems impossible. Do you think size matters in this case or it's just me, because I can't properly relax? (I would say the second one, because it hurts, so the size should be enough, so what could I do?)Sorry for my english