Just the knowing, not imagining, that you are no longer "in the loop" of your former workplace and the friends you have there. Oh, at first when you retire, there will be those occasional phone calls to "check on you," and such, but then one morning, the calls stop. All at once.
But you, being the lonely, unfulfilled friend, call them and at first they say that they are glad to hear from you and you chat for a moment. Then the next time you call, their voice is chaotic, nervous, as if you are hindering them from work and even say "I will call you back when this project is finished. I promise."
Then after a few days, weeks, the friend(s) do not call. And then you know, not imagine, that the promise they made to you is broken.
And you do your best to not think harsh of them. Maybe it was a serious project. Maybe they were "under the gun."
But the imaginations of one retired and now old, can be lethal. So let this post be a stern reminder to you who are nearing retirement what it is really like.
Keep your mind busy with good things and do not give any room to harsh, negative thoughts about your friends or neighbors.
And you will live happier and longer.