I remember feeling this way a "few" years ago. I was 16 years old when I lost my virginity but I guess the age doesn't really matter. The first time is what it is, be it at 16, 20 or 40.

I wish i could tell you exactly what to do, what worked in my case. Viagra was never an option. Giving up masturbation...well, I didn't, but it might be a good idea, temporarilly. Might help to be a little "starving" when the time comes. At the time, I spoke to a doctor. I described the situation to him and he said i was taking too long with foreplay and i was "worn out" by the time it came to penetration. Soon after that, me and my gf were able to do it. But I don't think cutting back on foreplay is what did it. I did, on that one time, but it never was an issue again. I think that what i really needed was to talk to someone, a little reassurance that i wasn't a failure , you know? You might want to consider talking to someone you can trust about it, someone who won't judge you.

As for more practical advice, try to get her to help you put the condom on. Let her take some initiative, let her keep you aroused, not just lying there waiting for it. Don't just get yourself hard and then immediatly jump for the condom. Know what i mean? Let her/ask her to tease you more and all that. You mentioned she was understanding of the problem, i don't see her raising objections to that.

It is said that "God gave men both a penis and a brain, but unfortunately not enough blood supply to run both at the same time." (yeah, Robin Williams. Sue me). I think that what is happening is your brain getting in the fucking way. Trick that bastard. It'll be mostly smooth sailing after this one.