There's some good advice there. It sounds like your girl isn't comfortable with the entire backdoor situation. You need to show her that you can get her off while going down on her, and teasing the ass. Maybe try half a finger. After a few times of this she'll be telling you to go deeper. Next you key on how hard she got off when you did that. Killah makes a good point when he says to get her drunk. When she's good an liquored up, tell her to imagine how hard she'll cum when you actually put the good stuff in there...then hold on.