From time to time we all have periods of being demotivated. I'm not saying that this is what is happening here, but particularly for people at school age, they'll often through times where they're still developing physically (it only really stops at about 25) that can make things a bit more difficult.
I find it helps to remind yourself why you're studying in the first place. A good education can help make sure that you're safe financially later on. You might want to make your family proud or be able to look after them etc. Or you might just want a ferrari and a mega yatch. They're all useful things to keep in mind.
Unfortunately motivation is just a feeling, and not one that we can rely on. In the times where you feel demotivated the best thing to do is harness discipline. I know that this is easier said than done and everyone has their own approach. Mine is a trick called 'Kaizen'. Essentially I'll make a list of tiny things that take me towards getting started. e.g. Open anki, open excel, put on lofi, say goodbye to friends on chat, clear desk, study for 10 mins.
By the time you're started you've done the painful bit. Motivation will eventually return even if you need to take a break every 10 mins.
You could change the kinds of activities to work the material to help with interest. Like flashcards, mind maps, powerpoints etc.
Oh and stay hydrated!