Hm, I understand it can be hard. I think we're all in the same boat here on this forum, eh?
I'm still trying to recover from Anorexia, and still sometimes partake in purging.
I have BDD, and what you're describing sounds to me like borderline BDD, stemmed from the fear of becoming fat.
It's 'protection' subconsciously. Best thing to do is talk it over with your GP, see what they say.
Also, as Lisa said, try to exercise normally. Maybe get a job that involves a lot of exercise (I'm a waiter, involves a lot of running around. Helps to no end) and keep yourself fit.
The more you burn, the more you can eat.
Maybe what you need to do is not try to lose weight, not try to gain it, but work out how much you need to eat/do to stay the same.
That should be a good starting point, lacking the guilt of losing more and yet stemming the fear of putting more on.
Hope this helps, eh!