I have also struggled with depression and a bi-polar disorder. I was diagnosed when I was 12 or so. I stayed on the anti depressants up until last year (4 years). The withdrawl symptoms were the worst thing I have ever experienced. It was horrible. But I actually feel better now that I am off of them.
It sounds like she's more bi-polar than depressed. Maybe switching her meds around to get the right dose and prescription will help. Maybe if you make sure she isn't taking more than she needs you might be able to keep the weight problem at bay.
Keep in mind that statistics say that more children than ever are being diagnosed with emotional disorders. It's great that she has found something that has helped her, and I'm not bashing anti-depressants in anyway, but maybe you should also consider other forms of treatment (if you havent already done so)
Hope that helps!