A different approach may be to find out why you did so well for a long time and now think you are not able to do so? This is not my expertise but why do you feel you are worse at your job now?
I also felt that I had not much more to achieve, and therefore that I had no challenge to solve. I completely disengaged and felt I had to hide the fact that I was just lucky/copying other people. That made me feel that I had to play politics to hide my shortcomings, and made me disengage even more from the technical side.
I feel it would be quite a bit of mental effort to get back on the technical side, but I also feel I would fail if I did so. Younger guys come out of university, work 14+ hours a day and are more skilled than me already in some areas.
Also, I started to have kids at about the time things started to go downhill, and I think that played a very large role: I became much less interested in spending so much of my time at work, focusing on making money and not helping anyone real...