OK. On the off chance that you are serious. Go see your doc or sexual health specialist. The posible causes are far too many to go into here I am afraid and need profesional advice. Not likely to get much of that here!
You sound younger than the usual age group that experiences ED, so I'd definitely see a doctor about this. If no physical problems can be found, there is likely a psychological component involved. Just because she's "hot" doesn't guarantee an instant sexual response if there are some unresolved issues between you.
Try not jacking off for a couple days, get some more foreplay involved, let her suck you off first, make out more, get rock hard then seal the deal, you'll stay hard!!