One of my favourite quotes comes from Mother Theresa: "I know God will not give me anything I can't handle. I just wish that He didn't trust me so much."
I don't know why this is happening to you. I don't even know if I believe there's any real reason for depression except that some people have the exceptional bad luck to be struck down by this terrible illness. I like to think that we CREATE the meaning behind our suffering. We take and learn what we can from every experience, good or bad, and this helps mould us into who we are. Whatever you believe, I hope you know you're not alone in your feelings and that you CAN get through this, even if it doesn't feel that way right now. Sending lots of hugs.