I know how you feel dude. I'm a 21 year old virgin who has never had a girlfriend. I had chances with 7 different girls in highschool and during that time I was a complete nervous wreck, my anxiety ruined every chance I got because I LET IT. Over the past few months I think my naïvety and anxiety regarding getting into a relationship has completely dissapeared.

I don't have much to say except this: take a deep breath, collect yourself and DONT let this chance go by FOREVER because you are experiencing a TEMPORARY problem. I think she will understand about your problems, but I'd advise just not telling her unless she actually asks about it. And if she does then I'd personally downplay it.

PLEASE don't let her go, do it for YOURSELF and I know it sounds weird but DO IT FOR ME. I can't stand seeing someone fail at something I've messed up multiple times.