Secondary School isn't easy, by any means. There's the constant pressure of having to study for an exam, keeping up with all the different subjects that you're taking in GCSE's and A-Levels, somehow maintaining a decent social life. It can be a lot. So the best thing you can do for yourself is to be as efficient at everything as possible and here's how to do it.
I was in secondary school for 8 years, did 3 years at university and it was only towards the end that I realised how I learnt and processed new information. If you didn't know already, learning can be split into 4 different ways - known as the VARK model. V = Visual, A = Audible, R = Reading/Writing, K = Kinesthetic. I'm a visual learner, meaning the best way for me to process is by visual aid. If I watch someone do something, I can remember every step straight away. An audible learner would only need to listen to the instructions being spoken out loud, a reader just needs the instructions written in front of them or they'd need to write down notes to remember, a kinesthetic learner would process and retain by actually doing the action. By understanding that I'm mostly a visual learner, I used that to my advantage and tried my best to visually represent everything; helping me retain new information.
My theory is that people are not exclusively one type of learner, rather they're a combination of two or more but prefer one more than the other. For example, I taught myself to read and comprehend new information but I'd still prefer diagrams. So I would suggest trying to explore different types of learning and see which works best for you then enhancing your ability learn to use the other types of learning - let me explain.
When you're learning something new, you won't always have a teacher/tutor to help you through the content. It's vital that you have the skill of sourcing and understanding new information on your own. Sure there are online resources like youtube to talk you through parts of the syllabus, but a textbook will almost always be your best and sometimes only option. So it's important for you to be able to read, comprehend and extract information in the written form. How you do it is up to you, you can record yourself reading it and listen back to it if you're more of an audible learner. Or you can read and try to make a diagram if you're more visual, whatever you need to do to understand and retain.
I myself need to oversimplify everything in order for it to make sense, but I have enough practise to do that regardless of how difficult the language may be in a textbook. Ultimately that's what it comes down to. Like any other skill, the best way to get better at it is to constantly practise doing it!