When I've posed that question to friends, I'm told that I don't need forgiveness because it's a medical condition that can't be helped, not a defect of character. Would I ask for forgiveness if I had cancer or a stroke? They also tell me that I would be forgiving of others, so I should be no less generous in forgiving myself (but again, no forgiveness is called for). That said, I haven't absorbed either of these attitudes, no matter how often I've been told. I think part of depression is denying ourselves the understanding that we ourselves would certainly give to others, making it more of a difficult process than it would otherwise be. I can't tell you this from personal experience because I suck at it, but try to be kind to yourself, just as you would be kind to others if they were in your shoes. You are no less deserving of it than anyone else. It's weird that in writing this I genuinely mean it - it's not empty words that I've heard - but I still can't apply it to myself the way I know I should. It's a process and I guess some people suffering from mental illness are further along with it than others.

This reminds me of some of the best lines in the movie Silver Linings Playbook.
Tiffany: "There will always be a part of me that's sloppy and dirty, but I like that, along with all the other parts of myself. Can you say the same about yourself, f-er? Can you forgive? Are you any good at that?" I love, love, love that movie.

Take care and love yourself because you are lovable, sloppy and dirty parts and all. It's more about that than forgiveness anyway, imo.