I think you are being too goal oriented and that creates anxiety.....
Decide with your girlfriend that you both will engage in a lot of foreplay going forward without having actual intercourse. Explore each other's bodies with your fingers, wet your fingers in your mouth and use different pressure on all parts of the other's body, smell armpits and groins.....I would say that at the right time, you would roll over and slip it in...HOWEVER...she has a hymen that will make penetration difficult the first time, and you have to apply pressure with your penis to break it.
I don't know....maybe you are eager to have sex, but not ready mentally yet? Wait six months.