WHY IS SEX SO PAINFUL?

I've been trying to have sex a few times this past two months and honestly...it hurts so much! Like he would put the tip in and it's all fine but any more than that is just too much for me. It's not that I'm nervous either. And he always eats me out so I'm pretty wet too. I just don't know how to get used to it without it hurting so fucking much!! 
Thanks for the help :)