Help getting it up tonight?

So tonight is prom, and I know my date wants to have sex, but I have trouble getting it up (and for that matter have never gotten it up when a girl is directly present) In my 3 attempts over the past year with 3 different girls, I have only achieved 50% max. I know I don't have ED, I have nerves, but I was wondering if anyone had any advice on what I should do. I don't have access to viagra, but would consider trying certain aphrodisiacs if effective. Even practicing getting it up now, I have much difficulty. Also, this isn't something I can talk over with my date, as she is not from the area, so this is probably a one time thing (and through friends I know she's expecting me to come through) anyway, I need to find a way to rid myself of my nerves, as I have tried concentrating hard and had no luck :/ Thanks