What Do I Have?  And Can I Get Better?

It's going on the 6th year now.  I cannot get back to a level of energy and good health long enough to get into a regime of daily exercise without getting worn down and sick for days.

I had mono at 18 years of age.  I was told Ihad it again at 21.  Then when I was almost 40 I believe I had the flu one night, fever, sweats, weak.  I pushed myself that night to do something for someone I thought I HAD to do.  I believe I pushed my resistence tooooo far that night.  It took me 4 weeks to get over the flu.  Since then whenever I exert myself physically for any length, I'm back to being sick:  weak, tired, headaches, no strength to do daily things.  I am on depression medicine, but even that stops working after a while.  

I want to get in shape, exercise and feel better.  Any suggestions out there?