9er gave you some good advice. The myth that sex--as in actual in-and-out intercourse--has to last forever is one that I'd like to see destroyed. Good sex is usually about a half an hour-plus of foreplay followed by maybe 10 minutes of the actual act.
If you want lube that lasts, look for silicone-based lube (not safe for use with silicone toys, but good for long sex sessions) such as Astroglide X.
I have the same problem your gf does--I just get oversensitive and it stops feeling good. One of the things we do is multiple gos in the same session with small breaks in between to let me...decompress, as it were. He comes, we take a little break, we have more sex, he comes, we take a little break, and then we go at it again. I know it's okay to go again when he can run a hand over my skin and I don't hit the ceiling
This is also one of those occasions when that silicone-based lube is excellent.