It should not hurt her to just have one of your fingers between her lips...does she use tampons? or if you havent tried using lube...do that. it should make things a little more slippery and easier. good luck!
I'd say nerves on both parts for sure. I suspect that she is nervous and a bit fearful of this so she is probably tense and that will cause her vagina to be really tight and not receptive to your penis. The lube idea is great as is alot of foreplay involving her vagina. I"m also not surprised that you're having some problems keeping erections...you're nervous too and now you're dealing with the fact that sometimes it won't stay up....and that only makes this worse. Now everytime you try there is some fear or concern on your part that you won't be able to perform...that is the worst possible thing for us guys to deal with! I think you both just have to keep working on this. Go slow, be patient, talk about things and calm down! You might try letting her be on top (although you're height probably isn't having any affect on this at all) but if she is on top then she can guide your penis in and take it as slow as she'd like. That way maybe some of her concern and fear will lessen. If you're on the larger size though in the penis department then you've gotta be really careful as well. OK, enough rambling. Hope some of this helps, let us know if you need more. Good luck and keep working on this...I suspect you'll both be fine!
Your height should have absolutely no bearing on your difficulty, unless you are trying to do it standing up. As for your problem, we aren't allowed to give how-tos, so you'll get none here, but I would suggest that your gf spend some time getting to know where her vagina is, and learn how to put something in it...
