Anxiety is likely the culprit here. Young guys with ED usually are suffering from mental hinderances, not mechanical ones.
However, to understand your situation more there are some questions I have:
1.) How often are you having sex?
2.) How much time have you spent with this girl?
3.) Did you have an erection during foreplay?
If and when you see her, don't act like it's a big deal. With my ex who I dated for a while, I couldn't get it up the first two times and then busted in <5 minutes the next two. If a girl really likes you, then she probably won't next you just because of that. If she ends up in your bed, just be honest with her and that will put you at ease.
However, to understand your situation more there are some questions I have:
1.) How often are you having sex?
2.) How much time have you spent with this girl?
3.) Did you have an erection during foreplay?
If and when you see her, don't act like it's a big deal. With my ex who I dated for a while, I couldn't get it up the first two times and then busted in <5 minutes the next two. If a girl really likes you, then she probably won't next you just because of that. If she ends up in your bed, just be honest with her and that will put you at ease.