Look up the Pomodoro technique. In other words, take 1-2 breaks per hour totaling maybe 15-20 minutes per hour. I do a simple 45/15 split when I can. I use a timer app on my phone.

You can test yourself on an hour per day with a free online course. Try MIT open courseware. You can gradually increase your study hours per day, but do not skip breaks.

I just restarted school myself... it's been a mess. If not for breaks I'd be toast.

Do not miss sleep. Do not miss sleep. DO NOT MISS ANY SLEEP.