Being inside your GF's vagina or mouth feels way better than your hand-not that I have been inside your GF, but you know what I mean
Even if you masturbate regularly the anticipation of being with her and being intimate with her after so long can cause you to pop your top really quickly.
When you are able to be with her try to arrange for longer sessions or, suggest to her that she give you a blowjob or a handjob
before coitus. That way after you orgasm if you lose your erection you can give her some really great foreplay and get her really close to orgasm before you penetrate her vagina. Tell her why you want to do it this way. Presumably you want to last longer in order to give
her more pleasure. She will probably be receptive. She might even tell you to not worry about it, just cum whenever you feel like it. In that case just enjoy. Once the performance concern is off your mind you might last forever-and if you don't who cares?
Oh, and if you start off with a BJ or HJ and are still hard after you cum take advantage of that lingering erection. Those can lead to some of the hottest, wildest, longest lasting sex you'll ever have.