Our sex just.. isn't working??

Hi guys! I'm seventeen and I've been with my nineteen year old boyfriend for over a year. No matter what, we just can't have sex. When we were first together he was on depression pills, which caused him to be unable to keep an erection for very long even if he was really turned on. However, he's been off them for nearly a year now and I'm really just confused as to why sex isn't working. Are we doing something wrong? Neither of us have had penetrative sex in the past, only oral. When we try, it's like his penis just bangs against me but doesn't go inside. I'm sorry if this sounds weird!! Does anybody have any advice or even a similar story?