I've read a couple 'self-help' books that indeed.... helped. When you're in that state of mind you sometimes need someone, whether face to face or via a podcast or book, to tell you how to get OUT of it.
Another would be to stay away from any device or media outlet (work doesn't count of course) and go see what man did not make... nature!
The best part of my days now, is that I don't have to go up too early in the morning, don't have to drive to work during rush hour, don't have to...
Only been retired a week yet, so I haven't started missing work or people... Feels like vacation :-)