Hi Crook. Please don't give up on physical activity altogether. Even just a short block (or whatever you can do) but done deliberately, is a huge accomplishment sometimes. Even just parking further from a store entrance can be a good step.
Seroquel has been very helpful for me, as well. For both depression and mania. My dose does go up and down a lot, though. I wish I could get down to 450 mg or lower again. At that lower dose, I can diet more easily. Diet is the main way to lose weight. It's far more helpful than exercise alone. I know, though, that if you're ravenously hungry, diet can seem impossible. I am almost incapable of cutting sweets from my diet, but when I can, I try to pick the lesser of the evils and to just eat smaller amounts, or just once in a day, if possible.
Are you on regular Seroquel or Seroquel XR? Though we're obviously all different, I have found Seroquel XR to be far more weight friendly for me than regular Seroquel was. I think it has to do with the action of the XR. Or something.