the should trap

My realization is that I have a lot of trouble with authority (nothing new, actually) and the reason why is that I hate being told what to do. Simple enough. Spent alot of time being told who to be, and so I resist it.
This is one of those classic "old defense mechanism that is now hurting instead of helping you" situations. Defensively, I fight back everytime someone tells me I should do something. Today, I find that even a simple injunction, even from MYSELF, such as "you have an hour, you should go to the gym" is met with FIERCE resistance. Any shoulds, from any quarter, are resisted. I sat in the truck after doing some bank business today and tried to explore why in the world I was having to try to force myself to the gym - hunger, distance and time were non-issues. Why wouldn't I do something that I like, which is good for me?
Gave up, came home and will spend the hour getting fatter instead. Truly a detrimental move instead of a constructive one. All for this defense mechanism.
Whatever I have managed to build and call my "self" is derived solely from rebellion and isolation. This tactic worked, as well as it could (which is not particularly well) in my twenties. But diffferent times call for different measures. I need a self that is constructive, not destructive. Friendly, not friendless. 
All this out of spite. Spite, anger and desperation have been in the driver's seat so long. Not sure I know how to power this rig on anything else.