You're too busy.
Trust me, I know. Every science tutor has been through the stress of school, studying, trying to balance job and a bunch of extracurricular activities, a girlfriend, etc. And wouldn't it be nice to sleep once in a while?
Here's a science tutor tip for you.
An economics professor named Gary North once said that we have to manage three things in life: Our time, our money, and our calories. In that order. Most people don't do this, or else they manage them in the reverse order and wonder why they feel overwhelmed by everything.
How to spend your time
So the secret of life is to manage your time. What does this mean? In the most basic, simplest sense, it means you make conscious, deliberate decisions about what you're going to do with each minute of each day.
On a practical level, you have to know what's most important, and set priorities. You should also keep in mind that certain times of the day are better for certain activities.
For example, two of the most important things I have to do are to create lesson plans and to write. I make sure I write and work on lesson plans every day, no matter what. Even if I have to get up early or stay up late, I take care of these things.
But since I've known myself for more than 40 years, I know that I seem to have the most ideas and creativity around 10 in the morning. I take advantage of this on the weekends and holidays by writing or designing lessons at 10 a.m. When school is in session, I make a point of jotting down ideas and outlines during nutrition.
Maybe you have a part of the day when you think more clearly. This is the time you should study your most important subjects. This doesn't mean the hardest subjects, but the most important. Let's say you want to be an actor or a screenplay writer. English is probably going to be your most important class.
You might be struggling in algebra. I would suggest you devote your best hours to English, even though it's not as hard as algebra. Reading a lot of literature will be more important to your career than learning to factor polynomials.
If you're in danger of failing algebra, it might temporarily become more important than English. But only until you've mastered enough to pass the class.
As you saw with the algebra/English example, priorities can and should change. If you go to your FaceBook page every 20 minutes, you probably won't be able to concentrate enough to absorb any serious learning. I know a science tutor who makes his students give him their phones before they start a session. (Aren't you glad I do my science tutoring online?)
But at some point your mind will reach saturation. For most people, this happens somewhere around 90 minutes to two hours. You won't really benefit from another minute of studying, so there's no point in studying anymore (it would be a waste of time).
When I get to this point, I either jump on my bike or head on over to YouTube. You should switch activities too. All you need is half an hour or so. Then you can jump back into something "important."
Pretty soon you'll find you can remember more, and get your homework done faster. Try managing your time. You'll finish things in a few hours that used to take a couple of days.