I gained at the very least 40-50 lbs when I was on Seroquel, Lexapro and Abilify. It's the Seroquel that I found to be the most difficult aspect of the medicine (I have dythsmia, but if I have a major depressive episode, I have BPD traits- for which I have been major episode free and med free for fiver years now- knocking on wood!!).
I was instructed to take the Seroquel at night, it's an anti-psychotic but also will help you sleep. Let me tell you- if you don't go to sleep within 20 mins of taking it, you can be prone to uncontrollable middle of the night eating. It was like smoking pot. Not only was I loopy, but I wanted to eat anything that wasn't nailed down!
Hang in there. The important thing is that you feel better. I know it seems like a vicious circle, because you feel depressed and your weight isn't helping. ( I also hope that you are in some sort of behavioral therapy or counseling!!) But hand in there