First time failure

Patient: So I am a 17 year old male, I had sex for the first time when i was 15 I have had a girlfriend for 3 and a half years and I have never ever once had a problem with getting it up. She broke it off about 2 months ago and one night I got really drunk and we hooked up i couldnt get hard so i blamed it on the alcohol. Then again we hooked up but this time i was sober and i still could not retain stiffness. Now i have been with a girl for about 3 weeks and when we start to make out i get an erection but as soon as we start to go farther it goes away and is extremley embarresing. I dont know what and it scares me soo bad I just want to know what I can do to prevent this I just want to be normal again