Welcome to the forum, Seahawk and Momma! I don't agree with you, I do not think he is spoiled at all. What you describe is perfectly normal and easy to explain behavior that has very little to do with whether you spoil him/her or not. Let me explain. You have three things going on here:
1. He/she is at a very difficult age because he/she is now sexually developed (no longer a baby or even a juvenile) and, although he/she might not be yet be at the sexual activity age (this will happen in a few months), if you have been feeding too much protein (free-feeding seeds or pellets will do it) and/or keeping him at a human light schedule instead of a strict solar schedule, he/she could be too hormonal.
2. He/she is clipped. This creates a terribly sense of vulnerability in them because the only way they have of avoiding danger is flying. When a parrot doesn't have the safety of the flock around him and is unable to fly, he suffers from chronic anxiety and clings desperately to the one element that makes him feel safe -which is you.
3. He/she is a GCC!!! They are one of the neediest species there is with the cockatoos being the only species that is needier than them. They are NOT easy birds to keep healthy and happy and that's why there are so many of them given out and abandoned by their owners. People see them small and think they are easier than larger species but they are not.
Now, how to solve this... There is no real 'therapy' or anything that you need to do different than what any other GCC owner would -and the ones that don't are the people who end up with screamers and biters. GCCS need a fresh food diet with lots and lots of fruits. They are mainly fruit eaters in the wild so their diet needs to be low protein and fat, and high fiber and moisture. They can't be free-fed any high protein food (this means that you can't just fill up the food bowl with seed or pellets in the morning and leave it there all day long). My GCC eats gloop accompanied by a leafy green, a fruit and a veggie (different one every day) for breakfast and all day picking and a measured amount of a good quality budgie seed mix for dinner (during the warm weather months, I add hemp seed and quinoa to their seed mix and sprouts and a bit of insect protein to their gloop) - my Codee actually loves to munch on a small piece of some type of tree nut, also so she always gets one for her dinner (and she goes straight for it, too!
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He/she needs to be kept at a strict solar schedule with full exposure to dawn and dusk. People don't usually know this but all birds are photoperiodic (a long word that means that they regulate their endocrine system and the change in the seasons by the length of daylight hours they are exposed to -if you want to do more research on your own, put 'avian photoperiodism' in your search engine and you'll see). They need to wake up with the dawn light (without artificial lights on) and go to sleep with the dusk light (again, without any artificial lights on). As the days get shorter or longer, their body registers the difference and the endocrine system turns on or off the production of hormones. This is very important because the endocrine system pretty much regulates everything in a body: sleep, appetite, mood, immune system, etc. But, for a parrot owner, it's even more important because this is what is going to make the bird produce sexual hormones ONLY when they are supposed to and not all year long. This is what makes them overly hormonal which causes them not only sexual frustration but also physical discomfort and even pain which, in turn, causes them to scream and bite. And, because too rich food is also a trigger for sexual hormone production, when you add both, you end up with a very frustrated and uncomfortable bird.
Now, the third thing is flight. I know you said that he/she needs to be clipped for safety but I am afraid that this 'safety' argument for clipping is not as 'solid' as it used to be years ago because there are plenty of lost birds out there that got lost while being clipped. And depriving a bird of the only mode of transportation nature gave them has deep consequences both psychological AND physical. On the psychological side, you have chronic anxiety and frustration. Nature gave birds only one way of avoiding predators and danger: flight. In nature, a bird that cannot fly is a dead bird so not being able to do this is extremely stressful to them (chronic anxiety). There are species that deal better with clipping... birds like amazons or macaws, for example, which are more independent so they don't need to be on their owners all the time. They are also larger and quite powerful so they don't feel as vulnerable as a little one when they live in a human environment. And they also tend to be perch potatoes so staying in one place for hours is not something that is completely contrary to their nature while it is for the little ones. GCCs are good fliers, much more flighty than the larger species and extremely clingy so, when they can't get to you whenever they want to (and they want to all the time!) and have to stay in one place, it frustrates them terribly. Then you have the physical side of it and the incontrovertible fact that flight is the only exercise nature gave them. Climbing doesn't do it, flapping wings doesn't do it, playing doesn't do it, nothing does it except flight (and this is not a matter of opinion but a scientific fact as there are studies that prove it). Add this to the fact that their respiratory system is different from ours so whereas we use our diaphragm to below our lungs, they need to fly because their lungs and air sacs are rigid (ours are very flexible) and bellowed by the same muscles they use when they fly. No flight = atrophied air sacs = much greater danger of getting respiratory infections (one of the main causes of death in pet birds).
I have one single female now (she has a boyfriend but he is a PeachFront) but I've had four GCCs altogether, all of them given up by their previous owners because of the same problems you have: screams and aggression, and they were all rehabilitated into happy, well-adjusted little birds which don't scream or bite (three of them have been rehomed and they are all still doing wonderfully well and mine is the sweetest thing possible!) so the recommendations that I am giving you have worked very well for me. The diet and light schedule are relatively easy to follow once you get into the habit but I know that you think that he/she needs clipping so I would urge you to not take my word for the information I am giving you and do a bit more research about it... you might find that you end up changing your mind.