When trying to lose virginity, i tense up my muscles a lot for fear of pain. is this what's making trying to get it in hurt so bad?

Partly: All circumstances have to be perfect for good sex: lock the door, light candles, start the music, & set your stage. You could be smaller than average and your guy could be larger than average -- either way, a lot of foreplay is mandatory. (your masturbating will help you to know your own body, so do that if you haven't). Use lubrication and be sure to have a patient gentleman in the bedroom.