Sex help!!

So I'm 20 and I'm technically still a virgin... I've tried to have sex with the same guy 3 different times and literally only him... but sorry if this is tmi.. I just can't take it. He really turns me on so that's not an issue but he just can't seem to get it in me.. he has a good size but I honestly didn't think size really mattered... any advice or personal experience that could help???