I wish i could mate but you're really the only one in control of the mechanics of it all if you get my drift. Maybe you just need to overcome performance anxiety? How you would go about that i am not sure. I can only relate your situation to most types of fear in which you simply have to challenge/face it in order to overcome (pardon the pun! lol). So i guess maybe if you guys were to just mess around a bit you know foreplay kind of stuff but be ready to do her missionary when you feel at ease enough to do the deed. If not then how about any other position? Does it have to be missionary? Alot of women get off on sex from behind because your more likely to come into direct contact with her cervix which feels great. I have a new boyfriend and he is teaching me a thing or two about positions and wow you know? Just experiment.. you're only young so just have fun with it try to relax and it will be fine. You sound like your girlfriend is more than willing to try some new things so go for it and enjoy yourself