My buddy swears by Durex Ultra Sensitive...and honestly, given the length of time he's been with his gf and the condom fees accumulated over the years, those things are apparently as effective as the Hoover Dam.

Anyway, you in large part answered your own question. You're afraid, and fear and anxiety are not conducive to sexual arousal. It's a bit of a vicious cycle...you worry about the condom breaking or the like, and your fear kills your arousal. The next time, you're worried about the same stuff as before PLUS the fear of losing your arousal. It happens again. Next time, same thing, except the fear of losing arousal is more pronounced. See where I'm going with this?

The best thing you can do for yourself is to relax. If the underlying fear is pregnancy, discuss the possibility of her being on a form of protection while you have a condom. Hamster's better with the stats than I am, but the chances of her getting pregnant while you're properly using a condom and she's on a contraceptive are absurdly low.

I hope things work out for you two, and soon.

Hugs,
Harley