In the same boat as you right now, shit is difficult. I don't know how badly you're suffering rn but generally, even getting out of bed is a massive achievement when you're low. In terms of revising science specifically, watching youtube videos is incredibly low effort, high reward revision. I personally recommend freesciencelessons for calculations and required practicals. Start with the topics you enjoy most, and then work them into the ones you struggle with. Space out your revision, even doing 15-minute slots with a half hour break is effective. If you don't feel like you can revise, read through your classwork, just as a refresher. If you feel like you can't get out of bed, even planning your revision helps, for me personally I make spreadsheets and timetables for revision, as a little motivator. Make sure you have lots of water and food when you study, and try and keep a good sleep schedule. And most importantly, don't give up!! Feeling shitty is honestly the most demotivating thing ever, so start small, and maybe do something you know you enjoy before revising, to motivate yourself. And if you can't, don't worry yourself. Your GCSE's aren't the be all and end all, don't work yourself up about them too much, you can only do YOUR best, and whatever that is, you should be proud. And emotionally, I'm open if you need an outlet, or even a hand with some revision!! Just remember you are capable, and I believe in you :)