I have worked with mechanics with ADHD. Drifting off or getting overwhelmed is common. Consider medication.

Otherwise, you know what you're doing, so break down the job by systems: Frame, wheels, bearings, shifting, brakes, bars. Create a list that works for you, treat each step as a job, take some sort of very short break between each step, come back to the next step fresh like a new job. Write the list down and reference it. That will allow you to concentrate for shorter periods without burning out or drifting off. Don't worry about the next thing until you're done with the one you're on.