I struggle with nearly the identical issues, except in my own profession (law), and for nearly the same reasons (good pay, family needs, original career goals achieved). Before I picked up GTD, I would have called it a slump, a midlife crisis (and I am approaching the Big 4-0), maybe "top of the mountain syndrome," where I have achieved my career goals too soon. Now I would say that I am struggling with issues/decisions at the higher altitudes, up around 50K somewhere, and I have been up here so long it is causing altitude sickness!
In 1990, I wrote a Covey-based mission statement -- "to be a successful partner and trial attorney in a respected medium-size law firm in my hometown." I achieved that goal approximately five years ago. Since then, I have been struggling with motivation/passion/interest at work, where my only "goal" appears to be to keep going for another 20-25 years. That goal is not enough to sustain me. And without the passion, drive, and motivation of a goal, I too have fallen from an "exceeder" to mere achiever. Where I once felt like I was working on meaningful legal issues, and acting as an outstanding trial attorney and zealous advocate for my clients ... I now feel like a cog, a tool for people fighting about money, leaving nothing in my wake except some type of dispute resolution and a pile of legal bills.
I thought that, perhaps, if I were more organized and efficient at my job, it would regenerate passion and re-energize my law practice (hence why I came here and started using GTD). But if anything, the GTD methodology has shown me that I have to make the hard decisions at the higher altitudes, or else I am wasting my time trying to implement any system. I remember a thread here discussing whether Covey is useful/effective and compatible with Allen/GTD, and perhaps I am Exhibit A for answering that question in the affirmative. Notwithstanding all of the terrific advice, information, and guidance I have received from the book and the members here about getting things done (special thanks to Katherine and Brian), I feel like I cannot actually move forward with GTD until I make some decisions and commitments at the higher altitudes.
So perhaps we procrastinate and underachieve because we know that, ultimately, we are not going to be satisfied with the outcome, no matter what it is. There is no emotional reward because we have no direction, no goal, no purpose beyond "stay the course." Without real decision-making at the higher altitudes, we lack the drive, passion, and motivation to actually get back to work at ground zero. At least for me, I have felt like I have been spinning my wheels in a career rut since achieving my original career goal. I am certainly not helpless or a victim or without alternatives, but it is far more difficult to modify decisions at the higher altitudes when you are a happily married father of three young children, rather than a 20-something student who got his hands on Seven Habits ... maybe I should have stuck with the first draft of a Mission Statement ... "Write the Great American Novel.").
Not sure this helps beyond comisseration ... but wanted to let you know that you are not alone.