You don't have time for everything. Therefore, you must choose what to do, but also you must choose what not to do. That is the hard part - choosing what to let go. Once you did that, the rest is (comparatively) easy.
Ideally, devote yourself to one subject and immerse yourself in it. Stay focused, refuse to do anything else. You'll have distractions, you'll doubt your choice, but don't start anything else until you finish what you've started.
If you have to do two things at the time, split your day in two. In the morning do one subject, then take a break (have a lunch, go to walk,...) Then, you study the second one. This break is important, don't jump from one subject to the next without it.
In summary, make your choice and stick to it until completion. Ignore everything else.