Can I ask my gf for my money back?
Why are you going to ask for your money back?
We rarely had sex and when we did, it was just awful
Holding lack of sex over someone's head and punishing them for it is just plain wrong. If you gave her that money you shouldn't expect it back just because you don't have "enough" sex.
It's a lot of money, I don't want to pay it
You are making it sound like she is a prostitute. This is deeply offensive, have you ever considered you might not be good at sex and that is why she didn't want to?