Welcome to PC. You are by far not alone. Many of us can totally relate to your story.
I have been suffering from depression since I was 13. I to was very active in sports and got good grades all through high school. Won a state championship in hockey in Michigan. Then in college things started going down hill fast. I got very depressed and skipped class all the time. Got a few decent grades but only in the classes I really liked. I ended up dropping out. I got into the plumbing industry and did very well. In reality I was barely keeping my head above water. All my energy went into work.
Read these forums. Get all the help you can get. If you have clinical depression it is not your fault it is a disease.