Been all of these. Used to multi task like crazy when younger. Turned out it was a coping mechanism for severe ADHD. Then had a few years of brain injuries.

Been all of these. Used to multi task like crazy when younger. Turned out it was a coping mechanism for severe ADHD. Then had a few years of brain injuries. I would see a problem. Say “reverse a string”, and think oh that’s trivial. A week later I would finally finish. I “knew” how to solve the problem, but as you described something was missing. Eventually recovered, plus got on adhd medication. Now I don’t multitask much. Only if I skip medication. I still can but it’s harder unless I’m off medication. In general I’m much more productive. But grander problems are a little harder as I’m not thinking too much beyond the moment.