Your situation sounds similar to one i had some years ago with a beloved GCC whose name was Baci - I named him after my favourite brand of Italian chocolates because when i got him he was so sweet, and it's Italian for "kisses" as well
I offer you my experience in the hope that it will help along with all the other great advice you've had so far, even to hopefully just provide that emotional circuit breaker that you need.
I had only Baci for a about 5 years prior to his premature death from pancreatitis in August 2018, so I do not pretend to know everything about them. What I can tell you though is that my previously sweet snuggly baby turned extremely vicious and bitey demon practically overnight at the age of about two. This was before I discovered this forum, and up to that point I'd had birds for whom puberty had really been a non-event, so I had no idea what had happened to Baci. It was the middle of a summer heatwave and I’d been working a few long day shifts in a row so he’d been stuck in his cage in a hot house for several days and I just thought he hated me for it. Like you, I was heartbroken and confused, convinced that I had lost my baby boy forever, he had become SO vicious, and thinking that he hated me and that the best solution might be to rehome him. I couldn’t get near him for days without him LACERATING my hands, and he was lashing out biting my ears and nose which he had never done before and drawing blood. In desperation I looked up all sorts of crazy things online to try to find a solution, and there’s a lot of stuff out there like the “earthquake” method (doesn’t work with green cheeks, it just makes them latch on even harder!) and dropping them suddenly on the floor (NEVER an option as it could obviously result in injury!) Bribery with treats didn’t work on Baci either, he would take the treat and then bite me anyway, and quite savagely too! I think I was also lucky that, although i never clipped his wings, he was an extremely reluctant flyer so he never actually flew to attack me.
What saved our relationship in the end was the procedure of "laddering". I would stick a few protective flesh coloured band aids on the parts of my hand most likely to get bit, thus lessening my reflexive flinch when I thought he was about to bite, and in turn lessening his reaction to my flinching. Then I would ask him to step up, and if/when he bit me, I’d ladder him onto my other hand and back and forth until he stopped biting, usually only a step or two or three until he stopped. Then I’d pop him down somewhere neutral like the back of a chair and walk away for a minute or two until he cooled off, then go back and repeat the process once or twice. Where possible I would repeat the procedure in neutral territory away from his cage, as green cheeks are also prone to a thing called "cage aggression" - another thing I didn't know about them before, where their cage quickly becomes their territory which they will defend to the death! Baci was a smart boy and it didn’t take him long to work out that he wasn’t going to get away with biting, which was pretty much the only thing I didn’t tolerate from him because he bit so dang hard! He’d grumble at me a little but I’d (carefully!!) give him a big kiss and tell him how much I loved him, then pop him down and he would go about his business quite happily. I also constructed what I would call my "armour plating" - about three bandaids permanently stuck together that I would leave within easy reach if I could tell that Baci was going to be bitey that morning, and I could strap them on quickly if/when the need arose.
Having learned a lot about GCCs from other members on this forum, I now realise that Baci's behaviour change pretty much coincided exactly with his reaching puberty. Undoubtedly there are better tactics than the "laddering" method that I've suggested, but it worked for us and as a short term circuit-breaker in this type of emotionally fraught situation I found it very useful indeed to help stop the worst of the biting, and it certainly saved me from having to make the heartbreaking decision to rehome. It helped us get our relationship back on track - I pretty much got my sweet baby boy back again and with much greater understanding of how to read his body language, how to avoid his biting and how to avoid it happening in the first place, so that it didn't become entrenched. So I hope something of my experience might be helpful for you too
You may not even need those, but they did help me in lessening my nervousness in approaching him, which seemed to make him bite even harder!
