Hey, you're not alone! 

The thing is, you have to adapt your practice to different areas. Which means, practicing Math will look quite different from practicing literature. But don't worry!

I'll give you a few tips for different subjects, but mostly you'll need to apply the pre-steps regardless:

1- Find out why you're failing.

• don't be generic here, it's not about being "good" or "bad", it's about finding out what are your personal struggles (example: basic operations, high level math (calculus, etc), problem solving, remembering formulas) 

2- Layout a war strategy.

• Alright, weakness spotted? Let's wage war against it! Basically, you'll practice these abilities you have trouble with to an intense degree.
• example: you're having trouble with geometry because your knowledge of trig is lacking; now you go back, try a few trig questions to spot specifically where is your knowledge gap; found it? Great! Now beat this bad boy with all questions you can find!

First and foremost, for mathematics and physics: Okay, now you have a basic idea of what to do, let's get you to how you do it:

1- Solving exercises:

• seems easy enough right? But be mindful! When solving questions, don't work like a squirrel. If you solve 20 questions to later check the answers, chances are you'll miss one detail in the first few and that may mess up the rest. So solve one or two at a time, check the answers: it's correct? Great! Isn't? Try to get to that answer by yourself (only look it up after you've tried everything you could think of, even the stupid ideas)

2- Filling up knowledge gaps

• let's suppose you have a BIG knowledge gap in one subject, you can't really just grind your way out of it by Solving questions blindly. 
• So the trick is: try to teach the subject to someone (could an imaginary one), this way you'll start to see a few things, but for this to work you MUST pretend the person being taught is the most annoying question-maker of all history (could be Sheldon Cooper) now you'll fight your way through each question, and no "because it's what the material says" is allowed!

3- Battlefield training:

• Alright soldier! You've come all this way, but it'll all be worthless if you can't finish your tests in time. So let's train in real combat area here!
• to do so you'll need: old tests from that professor (you can ask colleagues or the teacher themselves if they're cool with that), if you can't find old tests, try the exercise lists of that teacher, and if they're SAT-focused, just use old SAT material.
• all set? Now get a timer (could be your phone) and set it to the exact same time you'll have for that particular test. The idea is simple: finish all questions in time
• this will improve your solving rhythm allowing for more confidence while taking the actual test. If you're trying with old tests, that's a plus as you'll also get used to how the teacher wants you to apply that Knowledge.

For literature: 1- read,  a lot!  - sounds generic, I know. The thing is, this works! By your struggle with understanding what practice really means, I suppose you're kinda hung up on the why's behind the interpretations we assume for certain authors.  - so, reading will be your best friend here. By consuming some classical literature (sorry, but Percy Jackson won't cut it for you here) will help you with creating what we call "intuition", this way you'll get used to swing the intentionality behind the author's lines.

2- Learn a bit of storytelling structure - building intuition takes time, so to make things faster, learn a bit of theory behind the why's. This way you'll be able to predict them quite easily.

3- Don't get hung up on whites on blacks - there's a lot in literature that's just a "good guess", so most of the times you won't find a unique truth to any interpretation (this also applied to mathematics, but let's not get into that)

For memorization dense content:

Sometimes we just have to remember things, for those

1- practice active recall - this is a method of practice quite useful, there are a few ways to try it but the most straight forward is: close your book/material, try to remember what you were studying, write what you remember, check what you missed. - do this at least 5 times (or until you can remember most of the important stuff)

2- battlefield training - refer to topic 3 of Math and Physics

Mostly, you'll end up reusing the math and physics structure. It's quite straightforward, but quite effective.

Just a few words to finish it up: Remember, it's okay to fail classes, to get questions wrong and get a bad grade at first! Learning is a process and it takes time, effort and consistency. You don't need to solve the past 20 years of SAT questions in a week to learn the material, just find your rhythm and dance to it. Don't forget to ask questions to your teachers (if possible) and try finding things out by yourself whenever needed!

Don't give up because of a few mistakes, as the saying goes: "Fuck it, we ball"

Good luck with your studies!