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...I lost about 5 years once, from age 50-55, to a mysterious illness. Mine eventually cleared up, if that is any encouragement. I, too, use exercise as a mood management tool. I think a lot of people do. 20 minute ride and an exercise induced collapse afterward sounds like some sort of CFIDS or other similar condition. The research is mixed on these syndromes. But maybe you have something else going on. It's important to avoid those crashes, and endure the waiting using other mental management tools for depression.
There are a ton of books out there on dealing with something like this. Most of them are useless. This one,
Full Catastrophe Living, I found moderately helpful.