Start by fixing the physical pain. I started getting pain from sitting 12+ hours in a chair, too.

I improved my posture by taking Alexander Technique classes. http://en.wikipedia.org/wiki/Alexander_Technique http://www.vancouveralexandertechnique.com/

Then I improved my flexibility by taking up yoga. I've dropped 25 pounds since last summer.

Figure out the "mental" stuff and existential angst after you get physically healthy.