Even just go for a walk or a light bike ride. Daily exercise, even low-intensity exercise, can make a difference. You can ramp up the intensity later once you're feeling better, but for now start light.
I'm glad that you recognize that alcohol plays a large part in your weight gain. You'd be surprised as to how few people make that connection. The thing is, your body needs food, now more than ever. It never needs alcohol. I'm not saying stop drinking, because let's be honest, that's not going to happen right now. Try to cut back a bit though, and I think that you'll notice a difference.
Also, you recognize that your metabolism is fucked. That's good-- you know that the quantity of food isn't the problem. The thing is it is virtually impossible to quit a stim and not gain some weight. It's hard, especially if you've had issues with it in the past, but you need to realize that some weight gain is inevitable, and that your focus should be on getting healthy again. You're still very young, and you have TONS of time to lose any weight that you might have gained.
If you're not eating much, make sure that you're eating lots of veggies, and especially the dark green type. Something to try is to use the diet control aspect of ana, but focus on healthy eating rather than calorie restriction. Get most of your daily calories through whole plant foods, keep the fat fairly low (but not too low), and don't worry too much about protein. People do very well with only 5% of their calories coming from protein. Much less than this though and you'll be running into trouble. Aim for ~50g/day, not much less.
I became vegetarian as a way to harness the dietary control urge, and while I think that the last thing that you need is another change in your life right now I do think that it would be better for you to aim your dietary focus on healthy food rather than weight loss. Once you're fully clean and healthy again, your body will thank you.
The take home message--
the least of your worries right now is your weight. It doesn't seem like it, and my saying it probably doesn't make it any easier to realize it, but it's true. Get well, stay clean, and once you're healthy again you can worry about trimming back up. Not that you'd need much trimming I'd imagine.
Oh, and I'd suggest seeing a psychologist in the next little bit. You'd be amazed as to how much they can help. My biggest regret in life (which is tied to most of my minor regrets) is that I didn't see a psychologist years sooner. I could have saved so much time, started really living my life, so much sooner. Think about it.
Me? Thanks for asking

Yesterday was rough: had a mini-binge and got pretty sad about it. I'm still in a January funk, but I'm back on track at least. Thanks to some unadvisable... indulgence shall we say, over New Year's I spent last week horribly ill with a cold, and only just yesterday recovered my voice. Hopefully by the weekend I'll be well enough to get back to the gym, which I'm looking forward to since I've gained about 5 kilos over the holiday.