I guess that the hard thing is figuring out if the weight gain is:
1) Really a side effect of the medication
or
2) Part of the eating disorder
or
3) A little of both!
I don't have the eating disorder part so its easy for me to say that I'd rather weigh a bit more and be balanced, than be thin and actively manic/depressed...
Have you thought about getting help for the eating disorder so that you can be a little on the high side of your weight category and take your meds??
Hang in there,