No offense, but if this girl will leave you over that; then she's not capable of a long-term relationship. This is one of those issues that guys worry about; like girls worry about breast size and a couple of extra pounds. If you say anything you'll look neurotic and really freak her out. This is a time to let nature take it's course. Try ribbed condoms if she needs extra stimulation. Don't forget feathers and vibrators. You can rock her world with a little know-how. You might want to consider some of the instructional videos from the Sinclair Institute. Staying power is also crucial, and you can learn techniques that will definitely please her.
The number-one reason women fail to reach orgasm is lack of foreplay. I think good old-fashioned know-how will take you further than you may realize.