You have probably promised yourself you would study all weekend, only to lose the weekend to everything else. The problem is not your schedule, it is the size of the goal.
Small daily sessions work because they are easy to start and hard to skip. Twenty minutes feels like nothing, so you actually do it, and consistency compounds faster than intensity.
Set a fixed time and a fixed place, and attach the habit to something you already do, like studying right after your morning coffee. The cue makes it automatic.
Twenty minutes a day is about ten hours a month of focused learning, and that is enough to finish most courses. The people who finish are rarely the smartest. They are the ones who showed up daily.