I _always_ have this problem when starting a new relationship. Since I’m an older guy and have been through it a lot I just chuckle and tell the girl that ‘he’s shy at first, you’ll have to spend some time and get to know him, then you won’t be able to get him to go away.’ Then I lay back and let her play with me for as long as it takes. Nobody has ever been disappointed with the result and it makes her feel like she’s accomplished something.
In your situation just ask her to play around with you for a while without trying to make you hard. And by a while I mean 30 minutes or more. Watch a movie while you do it or something, just tell her exactly what you told us here about what you think of her and how your lack of erection worries you. Trust me, you’ll get over this, and you might even enjoy the soft fondling as well, I know I do.