I'm 18 and can only pull my foreskin halfway back?

This condition has a name: phimosis. See a urologist; it is likely you should have minor surgery to allow easy foreskin retraction. As it is, you're at increased risk of inflammation or infection under the foreskin (balanitis). And if you force retraction, it could be difficult to return, a potentially dangerous condition that can threaten blood flow to the head of the penis. Good luck.