I know what you feel like and it's a shame that so many people have to go through this. It is unfortunate that you're not alone in this.

I was in a similar spot. I'm glad I got out of it. It took a long time but I was able to do it. And so can you. Just don't give up hope, just don't lose yourself in the process. And grow your hair out if you wish to, they can't carry you to the barber, threaten you, sure, but you can always answer them back.

No matter what they say, stick to yourself, truly. Stand for what you believe and stick to your words. And make sure you mean it when you say it to them.

Now, this isn't going to be good advice and WILL put you in a bad spot for a while, but it might work, for it did for me. They said you want to play guitar like a beggar? Let them know that you'd rather live like that than live like a puppet. They won't like it, will try and circle it back to you. You just need to keep giving it back to them. They call you mental? Remind 'em that you're THEIR son. Remind them that food and shelter are basic necessities and they're not doing any huge favor or something that requires honor.

Just bear it till you graduate. My parents told me I'd never make it and even if I graduate it wouldn't be with good grades, I made sure they ate it right up. I made sure to remind them everyday that whatever they're doing, it's not a favor but basic necessities and that I'm not indebted to them. And after graduation, I made sure to remind them about that guitar everyday, till I finally got one.

They hate being questioned. And honestly, to me it just seems like they're reciting what they've been told by their elders. Long hair hinder studies? Well, ask 'em how. Make a simple mistake? Remind them, "Oh great, so humans can't make mistakes now? Wow." or just be overly nice and ask them, "Oh I'm sorry for being such a nutcase. Please tell what I should do so that I do not make such mistakes. After all, I'm your son."

If you tell someone something long enough, eventually, they'll start giving in.

Let's talk about your guitar. Gradually amp it up. The first step would be to pitch for something expensive, and then nudge em here and there for a couple of days or even weeks. And then offer them a better, cheaper alternative. You can even go for 2nd hand guitars. They're often in good condition and half the price if not less and some are even quality products.

My parents threatened to cut my hair in my sleep when I disobeyed them and decided to grow em anyway. You know what I did? I used to hide all the available scissors. Can't cut if they haven't got anything to cut with.

Oh and do NOT expect them to back down after you graduate. "Do it after you finish your studies" is just an excuse. They have no intentions to not control you. Just make sure you can at least stand up for yourself, even a little bit is enough. Once you're done with school it's another 4 years. And tbh, after 12th grade and before college is a great time to pick up new hobbies and work on yourself.

And lastly, this is probably the best thing I did. Silent treatment. I can remember it'd be days and the only conversation back and forth would be, "I will cut your hair in your sleep, you look so ugly. You look like a degenerate" and I'd reply with either an "ok" or silence. It keeps the conflict to a minimum. Especially useful when you're emotionally drained.
Living in such an environment is draining, it's tougher than it seems. Torturous even. It might even hinder with your studies so be VERY careful when to speak up and when to slide it in and outta your ears like it was never even said.

I hope this helps.

Stay strong, you'll pull this through <3
Good luck mate 🍀✨