Martin,
Firstly, my respect for being able to be open about your personal battle with mental illness. Its only when people are able to be open about it that it gets the recognition that it deserves as something that is both very real and has a significant effect on the lives of a sizeable chunk of the population.
I've never been a fitness junkie, but since I was about 9, cycling has been my favourite entertainment, de-stressor/release,mode of transport and general favourite thing.
In my late twenties, I became too busy and mobile (work wise) to cycle regularly. At the same time, and perhaps purely coincidentally, my mental health plunged. At 33 I ended up off work for 6 months through "stress" which was actually deep depression.
Nowadays, I cycle everywhere, whenever I can.
It gets me to and from work faster and more enjoyably than by car. It allows me to see new places at a pace that I can absorb my surroundings. It allows me to escape from whatever problems I may have as passively or aggressively as I want without endangering anyone other than myself.
Cycling is the best therapy I've ever had.