Sudden Weak Erections?

I'm 21, and about a year ago, for the first time, I couldn't get an erection with a girl I was seeing. I had had alot to drink that week and not much sleep as I had been away. I put it down to that but it destroyed my confidence, and in February earlier this year, I failed again, although the situation was possibly even more awkward.
I dated a girl for a little while in August time, and we first had sex drunk, but after that, the 2 months I was with her, everything was fine in the bedroom.
We broke up eventually, and I was trying to go back out with the girl I was seeing a year ago, just a couple of weeks ago when the same thing happened. I felt fine when we were messing around downstairs, went upstairs and got nothing. And it's not her at all, she's absolutely stunning and I fancy her like anything. Which is why it's SO annoying.
Since then i've noticed even in masturbation i'm getting significantly weaker erections than I used to, and it takes much more to achieve them. What's wrong with me!?? Please help, as this is really damaging my already frail self esteem.

Anything you can suggest is helpful and appreciated!!