That sounds deeply unnerving. This probably doesn’t apply, but I learned only a few years ago (I’m nearly 60 myself) that instances of remarkable cognitive lapses that I call “Swiss-cheese brain” are almost always a marker of stress (especially when I think I’m not stressed). Meditating a little or even general mindfulness (body scans, a few deep breaths) help enormously. Do with that what you will.

As to your game-playing take on this experience, I envy you your long and deep experience playing character-forward games. But I would imagine that GMing non-character-forward games would have to feel less interesting. Which makes me wonder: have you played Microscope, and does it scratch the same itch? It’s GM-less but emphasizes collaboration and narrative, though not necessarily character. Just curious.