I’m a 14 year old boy. There is this girl that really likes me and I feel the same way about her. We want to have sex, but we’re both uneasy about it. What should we do?

I have a rather embarrasing question but I really need help. When having anal sex with my boyfriend, after he inserts it and all it somes to a point where it is almost like it will NOT go any further. It's like his penis is hitting a wall of some sort and it really hurts. I am very relaxed before we start so the initial penetration is not a problem at all, it's just that there is something stopping his penis from going all the way in. what is causing this and what can I do about this?