I'm a virgin and I can't keep it up?

I'm 17 and I've been with my girlfriend for about 2 months. We are really comfortable with eachother now and I feel ready for sex, but we tried last night but I just felt nervous suddenly and I started panicking thinking I was gonna be really sh*t.. I tried to put it to the back of my head but I just couldn't hold an erection because all I could do is worry. I don't know what to do, is there any ideas? I'm worried that my girlfriend will start thinking its her fault when it's not, I'm just nervous. I need to get over my nerves but I don't know how. Please, any advice would be amazing. Thankyou in advance! :3