First off, the pressure will only make it that much worse. Being a guy, I honestly don't know how to tell you how to stop putting pressure on yourself, but I can attest to it making things worse.

Were you worried about the condom coming off the first time it happened? That could have started the ball rolling.

I've only lost an erection once and it was due to a lube we were using. It actually had a bit of a numbing effect on me. You might be having a similar problem with the type of condom you're using, the lubricant might be numbing you somewhat and causing you to lose it. Since the contact is numbed, you won't get it back until it wears off or something.

Another possibility is you're starting to lose it while putting the condom on. You might ask her to do it to keep up the excitement.

So yeah, I would start with mental concerns (like putting pressure on yourself and worry about losing the condom) then consider changing the type of condom or lubricant and finally I would try changing how you put it on.

Hope that helps!