When you say running would be too painful, do you mean any running at all? You could try starting a run/walk program, like Couch to 5K, where you alternate short bursts of jogging with running. It's a 9 week program, and it starts with something like minute and a half walking alternating with 30 seconds of jogging. You can repeat weeks/workouts until you feel comfortable moving forward.

My cousin was on Seroquel, and she gained weight. She warned me when I went on it, so I started counting calories immediately and put myself on a pretty restrictive diet. Fortunately, I was able to keep my weight the same. I was experiencing cravings like crazy, though, and saw how easy it would have been for me to gain weight if I hadn't been watching myself like a hawk. You could try a calorie counting app, and watch what sort of nutrients you're getting.

I know you're feeling desperate, and these solutions are a little slow moving, but they might be worth a shot. I hope you find something the works, and please be safe.