I think it can definitely be a part of depression, although major self-criticism can occur without any particular mental illness. I know for me personally, I am WAY harder on myself than others, which makes no sense and is not healthy at all.

PC has helped me with this in a way though. I like posting on my exercise thread and the "positive things you've done today" thread...it helps me to see the positive things I do, and to be proud of it. I also try to be gentle to myself on tougher days where it feels like nothing got done.