I can relate with you, Fuzz, as I've been in pretty much the same situation.
First step in picking yourself back up is going to be letting go of the past. If your ex keeps popping up at the places you frequent...find new places. She knows you go there, and that's why she does too. It makes her feel a little bit better about herself knowing she still has an effect on you...and from the way you said it ruins your night, it's probably blatantly obvious to everyone in the room. Letting go is the 1st step to getting over your depression.
Like Prod said, you're gonna need to get a control on your drinking. Alcohol lowers serotonin and norepinephrine levels...and when depressed, these are already pretty damn low. When you go out (to a new bar!), limit yourself to like a drink an hour or something. If you finish your drink before it's time to get the next one, order a water with lime. I do this because I feel like I need to have a drink in my hands at all times...
If you're burned out on going out, talking to girls, etc......then take a break. Simple as that. I know you're probably in a routine that involved going out on the weekends, but why continue if you're not enjoying it? A small break from it can do wonders for your mood. If you take long enough break, you'll get that hunger back...the hunger to tackle that club full of hot girls like a predator to it's prey.
Don't be afraid to seek professional help with this, either. I was doing all I could to manage my depression and similar emotional issues, but it wasn't enough. Finally, family convinced me to seek help. Turned out that I had more than depression. I'm also manic bi-polar...if you can't do it on your own, it's worth checking in to...
