Thinking hard is fun for me. The problem is my thinking stops after a while. It's very frustrating. I want to keep thinking, but I can't generate anything useful. This sounds consistent with the idea of toxins building up in the brain cells.

Not feeling pain from overthinking sounds like subconscious masochism. Either way, I almost always take breaks, and enjoy them, usually a lot, temporarily erasing my frustration for maybe an hour.