Personally, I am concerned that you are pushing yourself so hard so soon.
While you may not have any physical "problem" as such (i.e., heart condition or disease) I think it is still possible that you have had an "overtraining" condition, something which the doc's may not have a lot of knowledge about or may not have diagnosed.
I think you need to get back into riding rather gently and easily, with the long-term goal of building your body and endurance back.
Also, you might like to try finding other values in riding besides the endorphin and adrenalin rush and other things you get out of riding. Such things as the quiet and peacefulness, the camaraderie, etc. Otherwise, as you grow older, and it is more difficult to get that rush, it is even possible that you may just stop riding, as others in your situation seem to have done.
Good luck!
From:
http://www.ultrafit.com/faq_general.html
A: Overtraining is a very individualized phenomenon. There isn't one way to come back from it as it depends on a number of factors, not the least of which is how far overtrained you were. I would say it takes at least 3 weeks to reach that state. At this level, if caught early, you should be able to take a few days off and spring back easily. Most people never catch it that soon, however. If it has been built up in 8 weeks or more of overtraining, you may need 2-3 weeks of recovery time. If more in the range of 12-16 weeks, it may take as many weeks or more to spring back.