Talking to the girl, letting her know it's not her, that you may just be stressed, and such is a good start. If it doesn't come up, don't make a bid deal out of it, it will happen soon enough. Just have fun.
Also, just concentrate on her. Don't think about you, your erections, getting hard or anything like that. Just work on pleasing her in any way that you and her like, that doesn't involve your penis. It may do two things for you. One, you still had sex and are fulfilling a relationship need even if it didn't involve penetrative intercourse. Two, hopefully it will automatically relax you because you're not in your worrying mind and the act being sexual will give you an erection.
Just don't make the act about getting an erection. If you get one, good. If you don't, you still had a good time and there's always next time.