I finished my HSC only last year, and school was extremely hard for me motivation wise. I hated my classes, classroom setting and having no real control cause it's something we feel like we're forced to do. So I completely understand as someone who does love learning.
15 was very hard for me mental health wise. My advice to you is to realise that whatever grades you get now won't be detrimental to anything, trust me. If all you can manage is passing your assignments right now so you can take some steps back, then that's completely okay and reasonable. There's extreme pressure on academic performance in year 9 and 10, and looking back on it, it really wouldn't have changed the marks I got in my HSC and it's ridiculous.
Though it's easier said than done, especially if you're someone with performance anxiety, please take breaks - I'm a guitar player too, so try to remove the pressures of success and enjoy music. Talk to people and be around friends and family, play your favourite games, whatever puts you in a happy place, try to do it more often.
I had a bad period of depression at 14/15 where school was completely pointless to me and almost unbearable, I got away with getting B's and doing the bare minimum. I completely understand your frustration. It helped me to look long term into a bigger picture. I've just started Uni to do a psychology degree where I hope to work in mental health and help people with the same things I've been through - you will be at a place in your life where you have more control to enjoy learning and not feel so restricted by school. Because trust me, what you're feeling isn't your fault, it's the circumstances you're in, and you won't be in them forever.
Your mental health is the most important thing despite the pressures around you that make you feel like your wellbeing isn't a priority. And we're here to remind you how important and valid your struggles are, especially me as someone who is so happy to be out of school.
I personally went to my school counsellor who was amazing - I could get out of class and go to her office where I could process things and get some great advice. I've also used headspace services and would really recommend them, even as someone who was very daunted by the idea of it. I know you're frustrated with talking about it, but in doing so you will get a lot of advice on how you can change and manage things.
Thank you for reaching out, I hope I've helped in some way.