May be this would help you get better results (from my humble, little and incomplete experience) :
1- First, try to understand the definitions or formulas of the course (the theory) :
concentrate in the class, no chit-chat, :nervous: no silly distractions.
2- Try to understand with examples :
you should get the help of your teacher or a classmate who already undertood. Ask as many questions as you can, and be as much annoying as you can.
3- Do as many exercices as you can :
You know in some subjects/chapters you won't fully understand the course until you do enough exercices. You have to do some effort, you won't regret it.
4- Every time you find difficulties make dua and ask help from Allah SWT.
Remember Allah wants us to seek knowledge everywhere, and He will help us.
5- Sometimes you can't understand something directly, but things could be clearer after some time :
(after taking a nap, or after making a walk, or after making a cup of tea:rollseyes) , so take your time to understand, and don't be depressed if you don't get something very quickly, because you probably will get it after reflexion or after some time.
6- Don't underestimate yourself.
You are able to understand and excel in any subject (well everyone with a brain will do). All you need is to concentrate on that subject and give it what it needs from your time and your effort.
7- Don't study everything at the same time :
Make priorities, organize your schedule, start with global concepts/notions and then try to treat the details of every part. Tune your study with the amount of time available : If you have too much time, then try to understand every detail and try to make to many exercices to repeat and confirm what you understand. If you have less time, try to get over the most important notions, try to understand the general concepts, and just do practice of what you need (+/-3 exercices for each Chapter/Part for example).
8- Don't concentrate only on the marks you will get,
Concentrate on the knowledge you will get and the benefit of this knowledge in you life and in your job. Belive me, students who only concentrate on marks and don't try to understand the essence of every subject , they will have too many problems in their jobs later. I know many engineers who face difficulties in their jobs because they don't master maths or physics, while they had very high marks in school, and when you ask them about that, they tell you they forgot all they learned in school, because they didn't know the usefulness of the subjects they studied and they only memorized some formulas and memorized only some types of exercices that they repeated in the exams.
So try to understand what is the subject about, how it can help you in your life / future job, what are the main concepts of the subject that you have to memorize forever.
9- Do revise and repeat what you learned :
You will be surprised how fast we can forget what we thought we fully understood and memorized. So revise periodically (every 3 to 5 days).
10- Don't revise everything the night before the exam:
You probably experienced this. What do you think about it ? How many times did you regret that you have not enough time to revise a subject, and that you could get better results if only you had just more time :grumbling.
So prepare yourself before exams approach. Think everyday about the exam (no matter how far it is). Make periodic revision, so the night of the exam you will be prepared and you will have only to look quickly at every subject and check your understanding.
And as I said, the scientific subjects you mentioned need practice (not only memorizing like other subjects). You have to do exercices bro. Try to find as many corrected exercices as you can. It's up to you to make yourself good or weak in them. It's only with effort.
I've been trying to find an easy way to understand Physics, Biology, Chemistry and Add Math but it's just isn't working. These are the four weakest subject and I've been failing em since I took them up.. I tried to really focus and study but I end up flunking every time 
. Now I'm just a few months away from one of the most very important exam of my life, will I make it??
Inshallah