As a bipolar dude, I've found that when it's hard or impossible to do my regular, intense exercise, it's key to just do whatever I can do at all, even if it's absolutely minimal.

Too depressed to lift heavy weights? Try to lift light weights. Too depressed to lift at all? Try to go for a walk. Too depressed to go for a walk? Try to get up and shower.

It may not be much, but doing whatever I can and being proud of myself for doing it helps a ton.