I can't have sex with my girlfriend, and it's very frustrating

I'm really desperate to get some help, so thank you! To give you some context, I'm 25 yo and have never been in a serious relationship. Even more odd, I have never been liked by the same person that I like (and viceversa). That ended about 5 months ago, when I met this amazing girl... Literally, the girl I've been dreaming of for so long, and now she's my girlfriend. Due to the fact that she's almost a virgin, sex still hurts for her so she's very scared and cautious about trying, which is perfectly fine. She has wanted to try a couple of times, and I get very nervous and anxious about not being able to make it feel good for her and not hurt her. The problem with that is that, although I'm very aroused and fully erect when I'm with her in that moment, as soon as she tells me she wants to try and I get the condom I lose my erection and just can't get it back until the moment is gone. This is extremely frustrating because I can't seem to get over my anxiousness and be able to do it. Does anybody have any tips on what I can do to help me get relaxed? I'm also very worried about what she's thinking about me now...

sex is not supposed to hurt (unless she's minuscule and you're huge) :) she must learn to relax and lean in - give in - open up (you can use lubricant if necessary - wetter is better) you both must learn to take your time : slower and slower even is the best - get into the feel - breathe in - take ALL your time she must not let it all rest on your shoulders : this will of course make you become anxious and fail erection - she can participate if you feel you have no time - of course you will be all stressed out : you have hours and hours to make love :) making love is not only about penetration : it's all the other stuff too - the kindness - the sweetness - the playing - the talking - the fiddling around - the laughing don't put her on a pedestal - and then wondering what she could possibly think about you : that's not putting both of you on an equal footing communicate: you must be able to talk openly about anything