Last time, when studying for my exams, I would use a alarm clock, set it to an hour or two and "force" myself to study undisturbed during the specific time. After which I'll reward myself with a short break. I keep repeating until the entire chapter is completed.
Pretty effective 'cos it breaks down the big chapter into small sections with a deadline, and i'll usually persist. Maybe you can try that too, set a timer of an hour or so, work on your drawing. Don't stop until the alarm rings, then have a break (go get some inspiration).
Would be good to try a hybrid of methods and choose the one that's most comfortable for you though
