What to do?

Me and my gf have been together for a year and we have been sexually active immediately after dating (we are teens). We have been in bed many times and i really enjoy doing stuff with her but theres one problem. We have tried to do the deed multiple times but mine wouldn’t fit in her yk what and it pains her when i try to slide it in. We have tried using lube but it still wont fit in. What should we do?