Lots of things can make you unfocused.
That's because you are more focused on your better plan.
Your mind is turning gears on how to complete the task in a better way then you are instructed. It's a good thing.
That doesn't sound like ADHD. It sounds like you may have a mild form of Autism. But you need a medical professional to make that determination.
Not even close.
Only a doctor can be sure. You're not going to be able to diagnose or treat yourself.
No, not at all. I know autistic people. And you sound like you may be autistic.
Talk to your doctor.