Swim skills are complex. With instruction adults often make more progress in hours than could in years on their own. Limit skill drills to 20-30 minutes. Most of the positive imprinting from any drill takes place in the first 20 minutes or so of practice. After that, even the most attening pupil's mind wanders.
Execution falls off, and bad habits creep back in.
Work on one aspect of skill at a time. For instance, you can concentrate effectively on extending your hand farther in front or on rolling your hips as you stroke, but not both at once. Better to do one thing really well for 20 minutes, then something else not requiring great concentration for 20 minutes, then a third 20 minutes concentrating on something else again.