I don't know if this will help in your situation, but I'd suggest the same approach as Wolf. Lots, and lots and lots and lots of foreplay. Sexy kissing and touching, whatever turns her on, then back off. Perhaps most of the day like this. Go for lunch, then more forplay. Maybe kissing in the movie theater, then more forplay later. I mean, pretty much tease her most of the day.
And if you can have the discipline, once you are having intercourse, try stopping well before she begins to become tense or apprehensive. Go back to forplay... whatever it is that gets her most turned on. And try a short time of intercourse, but stop while she is actually feeling pleasure. Let her relax and feel safe, and back to the forplay.
I agree with the previous post that mentioned letting her always feel in control (at least until she decides she might like to give up a little control, but that might be a ways down the road). In my opinion, if you try to have sex, and it always progresses to the point of pain, you are both essentially practicing that scenerio. Her body will anticipate the pain, and it will happen more and more quickly.
I know this sounds really terrible, but if you think you can, finish with your own hand (or if she's willing to do oral) rather than let her metally get to that 'bad experience'. She might need to reherse or practice 'good sex' or at least 'sex that doesn't hurt' a few times (many times?) before she can get more relaxed.
So maybe try a long day of teasing, followed by alternating thrusting with foreplay. It might even work to alternate thrusting with just keepig yourself still inside her for a few seconds and kissing until she asks you start again (or starts moving her hips). But I think it's in both your interest to slow down and get her relaxed BEFORE she reaches the point of apprehension/pain/tension/anxiety (anything negative) cause you don't want making love associated with negative stuff, or it will be an even longer road undoing that mental association.
By the way, I was very touched by your sincere question, and your obvious compassion for your girlfriend.