I've got 5 years of early retirement under my belt, and I feel like I've had the inverse experience of the OP. The first two years were an incredibly difficult transition period for me.
Losing the status of being a higher up at a successful (small) company was extremely painful, especially because I didn't realize how much of a perk of the job it was. Like many others in this thread, I realized work provided a venue to solve interesting problems with interesting people, and I knew I would miss that (but also knew there were other, non-work ways to scratch that itch). Looking back, though, the ego/status aspect was probably the biggest benefit, and I never admitted that to myself.
Fairly soon into retirement, I made the choice to address the issue by decreasing my ego needs, rather than pursuing another high status non-work position. In some sense, it wasn't too hard to do (though I did find that I had to give up programming for a while, since it turned out that, despite my love of math and puzzle solving, the real biggest draw of programming was the megalomaniacal sense of being a god in a universe of your own creation). The issue is, I went too far, and experienced pretty significant depersonalization, which, from the inside wasn't terrible, but I think made me too weird to interact with my family and friends.
I've currently accepted more ego back into my life (hence this self-centered posting, for instance), but kept it below the old levels. I'm happy with my life, and find it much more even keeled and comfortable than when I was working, though I still miss the emotional highs of succeeding in a big work project.
Anyway, I think what really got me to log on here and post this was seeing the various other posters claiming work as a source of meaning and maybe poo pooing the OP, a bit, for giving it up. Now, on the one hand, I'm not really disagreeing, but I just wanted to add the nuance that, for me, turns out most of that meaning was not coming from nice pro-social things like cooperating with a team and working on interesting problems, but mostly just base status drive and ego. Now, I don't know how much my experience applies to the general population and most jobs, but I have a feeling it probably does apply to a fair chunk of the readers of this forum.