Problem with my girlfriend- sex is not fun for her?

She is not ready: Sex is great when both partners are mentally and physically ready. She is not ready. Try other things, cuddle, foreplay without sex, find out if she has had a bad experience in the past, is she in pain? Do not put your penis in her until she wants to have sex, you will only make it a harder problem to fix in the future. Be her friend first, let the relationship get better before you try again.