Oh my, I have found my people! While I didn’t retire at a young age, so can’t speak to that particular piece of it, the rest of what you’ve described really resonates with me and the ability to dive deep down the rabbit hole of a new interest is a big motivator to retire.
That said, I go through periods where nothing sparks. I’ve found a few hobbies that have stood the test of time, mainly because there is just so much to learn that I’m not sure you ever truly master it. But even those wax and wane. Personally, I had to escape the cold, gray winters of the Midwest—that for me was a guaranteed slip into depression.
To get out of the slumps, a few things come to mind. One is forcing yourself to just start a new project or hobby, even if you’re not feeling it. That, interspersed with lots of long walks or runs in the sunshine, helps.
You didn’t mention travel at all, but getting out of my element in a physically active way always gave me a jump start. In spite of being an INTJ, we still need social interaction. I may be reading between the lines, but you sound very socially isolated, which is also a road to depression for me. One route might be to book a group adventure travel type of trip. I’ve found kayaking trips to be great for this because you’re outdoors, physically active, get lots of alone time, but can still have a somewhat social experience at the end of the day. There’s a fantastic group that runs one in Belize if you’d like a suggestion
Another thought would be to take up a sport that is solo, but somewhat interactive (Eg tennis or surfing).
Art/maker type classes can also be a good outlet, though I usually end up just wanting to build a studio in the garage... It was mentioned above, but fwiw, pottery gives me the feeling you described with the fence posts. And the great part is that it’s a hobby that is very complex and very difficult to master, from throwing, to glaze chemistry, to firing—lots of rabbit holes to learn about!
While you said you weren’t interested in volunteering, I too thought of habitat for humanity or ngo work. It sounds like you need to get out of your head and mix things up a bit. The latter could be especially helpful. Feeling useful/helpful is a big booster for me—I also love to teach, so probably a tie in to what evrclr mentioned above.
I might caution against a YouTube blog or that type of thing, only because for me, the more internet/screen time, the worse the slumps get and the more inward I focus.
This is a really interesting thread and I’m glad you posted. It’s helpful for me to hear from others with similar traits and it’s made me reflect back on times I had that flow vs not.