Its not unusual for girls to experience some pain or discomfort during sex. Especially in their teen years. But for a boy, no. It is very unusual for a boy to ever experience any type of pain or discomfort during sex.

You are experiencing pain because you are suffering from phimosis (tight foreskin). It is supposed to be fully retracted for intercourse.

Your gf will be receiving little or no stimulation during sex because your foreskin is covering the glans rim which ought to be in direct contact with her vagina to give and receive full stimulation. She will only feel the discomfort of your thrusting.

You need to get yourself circumcised to cure the phimosis and allow your gf to enjoy sex.

Well if your foreskin has not released then that could cause some pain. Normal age for detachment from the head is 15 to 17.

You may need to use personal lubricant.