I've been in a very similar spot. I was spiraling downward and thought biking would help, so I started riding July 5th of last year; I commuted every day until November when I got the flu.
Just as I was recovering from the flu, I had a secondary temporary nerve damage issue that really sapped my ability to think and move. It took me a month and a half to recover from the flu. I was 25.
Then, the very day I strode out to demand to see life's manager, the ground was icy and I bit it while rolling downhill at 15 mph. My accident was so bad I ended up needing therapy for the PTSD it caused, even after my face and shoulder had healed.
Biking was supposed to be my ticket. It was supposed to make everything better. But it didn't; not on its own. Bicycling, I discovered, was really a symptom of my inner desire to feel better. In the end, I think that's what worked for me. I recognized that part of me wants so badly to feel better that I'm willing to go out and ride my bike all over the place. If I'm willing to do that for myself, then, I guess I'm not a total write off.
After 12 years of being depressed, 10 months ago I went to see a therapist for the first time. I still see my therapist every week and it's easily the best money I spend. Probably the only advice I can freely give is that you should find a rock-solid therapist. I am constantly learning about myself through probing of corners of my mind that I've always taken for granted.