Ushijima

I’ve got this advice from a book Reddit page and it does work. Generally when you’re tired of a medium (eg books or manga/hwa/hua) you should switch to stories that may interest or resonate with you in another medium. This gets rid of medium fatigue and you can return to the original medium in time. Not only does this rebuild interest but also allows you to engage with interesting stories across different mediums.

I think that's normal, everyone has phases that come and go I think. If right now you're more into novels or gaming or whatever, I think that's just the way things go sometimes, our preferences change with time. Maybe you will be in the mood to read manga again in a few weeks or in a few months etc., maybe it won't be like before (which is where I'm at rn), but that's ok, either way you will find other things you enjoy, so don't worry too much. However if you feel that way about every hobby that's a different story.

is it like the SAT? the math I mean

Find the maths book for GED and write down the topics you need to recover. Then, watch YouTube videos about those specific topics and then read the maths book chapter by chapter while also solving the questions in the books and doing past papers you find online to test whether you understand the topics well. The same goes for Science. As a student who just messed up my Maths exams yesterday in IGCSE, I would advise you to practice maths every day to not fail terribly like me.

(I left a comment but it disappeared:))
So, basically find the maths textbook you would need for the GED curriculum and note down the topics you might have to cover. Then, go on YouTube and watch videos related to those specific topics. After that, read your maths textbook thoroughly while solving the questions included in the textbook and do past papers online to familiarise yourself with the actual question formats. The same goes for Science.