With all due respect, if your depression was indeed sin and not medically induced, I'd like to suggest the possibility of it being the sin of unthankfulness. I am persuaded that unthankfulness is the key ingredient behind all depression. Repentence and thankfulness to God are the true remedies. I know it's a hard pill to swallow when dealing with a crisis such as cancer, but even then, a grateful heart inclined to rest in the Providence of God will vanquish depression.
Well, thank you for that judgment.