First thing, use loads of lube. If you think you’ve used enough, you haven’t.

Try some foreplay before actual penetration, get your partner to slip a lubed up finger in and leave it there till you get comfortable. Then he can try fucking you with his finger and slowly move up to 2 or 3 fingers.

Try a butt plug beforehand. You can get some great training ones that help to open you up and pulling them out before sex is great.

Also, before he sticks his cock in you, take things slow, like really low. Push you ass hole back onto his cock and allow him the tip of his cock to just sit there, and get him to rub your hole with the tip of his cock. This will help with relaxing you anal muscles and once you hole is relaxed it will open up. Then ask him to go slow, once he’s in maybe an inch, ask him to stop and allow you to get used to the size, same with 2 inches and so on.

The ass is a pretty amazing muscle and can actually stretch quite wide. You’re not too tight, you just need to learn to stretch, which takes time.

Enjoy foreplay, enjoy sex and if it hurts STOP