Is it too early to be burnt out?

[bleak zealot]

I understand how you felt, infact I have similarities with you.
I suggest taking a short or power nap, perhaps 25 or 30 minutes instead of long hours as it can leave you groggy. But if you're fine with long hours, then it may not be a bad idea. Rather than scrolling on your phone or playing games, sleep is a better alternative, except for if the nap disrupt your sleep cycle,
And then after you woke up refreshed, you can go on with your life again. You can take a shower, have a nice dinner and finally, you can study again.
For the homeworks, maybe you could balance out 50/50. Or finish all the homeworks for the night and review another day. But you can take this as practicing instead of doing homeworks. Also, it's important to have a clear view of what you have to study and revise for each subjects. For example, Physics (Read Chapter 1, practice questions). Having a daily schedule helps too.
Procrastination is common in most of us. Not only kids, but also adults. I suggest cutting down distractions and building up your discipline and mindset will help.
IGCSEs can be stressful, especially with all these lessons to cover and all those pressure. But know that you're not the only one who's going through this, many other people are also striving through this and have succeeded so I believe you can do this too. It's important to stay positive even if it gets tough, remember, a progress is still a progress regardless of how slow it is.
It's actually honorable how you are different from others. You have a strong reason and you have a strong mindset to achieve this, so go and ace it. Everything seems impossible before they are done. One day you'll be looking back at these times and will be proud for not giving up.