Foreskin hurts during sex. I'm circumcised.

I am 16 years old and so is my girlfriend. We have an amazingly strong 8 month relationship with absolutely no issues. We were starting to have sex earlier and I couldn't completely enter her because my foreskin was pulling back and it was painful. I don't know if it has anything to do with the fact that well, my girlfriend is tight. And I do have a fairly large penis. I have very normal foreskin while flaccid it seems. but when I'm erect, it doesn't feel uncomfortable, but I cannot pull my foreskin back much at all. And while I was entering her my foreskin was being stretched too much and I couldn't go through with it. Please tell me what to do, how to fix, or anything to help. I will give any additional info that you need, just ask. And I've already searched reddit for this and I couldn't find any similar issues.