Chad,

Don't beat yourself about the past. Easier said than done. It's often the beaing ourselves up that starts us down a negative spiral. Be proud of yourself about that cover letter. I think they are hard and I always cringe at the though of doing one; I haven't done one for years.
As far as your question, I have had depression since I was a young teenager. Major Depressive Disorder during certain parts of my life but definitely dysthymia throughout the entire years. I don't expect to ever totally get rid of all depression, but when I feel OK, I do hope to at least to get a steady level where I enjoy life more than not.

Z