As a person who has broken other species trust , a few times in our lives, yes you can regain trust.
It takes as long as it takes, and you have to start over earning the privileges you once had.
My worst with my GCC , took about six months to recover.
I'm dealing with an unknown cause of fear of hands in my quaker. It's taken about 20 days to get back to taking treats with out bites. About 24 days in , I can pet her her again. I have managed 2 step ups but with small bites still, but not ripping me to shreds...I am going to wait longer to work on step ups again.
As general non CAG specific, be predictable, use same phrase, a heck of lot of just handing treats and saying hi. I do non contact targeting , I myself don't use clicker or stick , I just tap stuff and ask them to walk over for a treat , touch or pick up objects for a treat .
For already established screaming, I found interrupting the moment it starts and redirect , then do a simple thing to earn a treat . For my non greys they don't link the treat to to scream , because at first I move them , then I have them do a trick then I give a treat. Then I usually move them again and set up with foraging..Trying to prevent known screaming triggers from happening. If like mine screaming starts after breakfast is finished and they go to a certain scream spot .I instead have them do training after breakfast, then move them to a different perch with something to destroy. Then after they lose interest in that maybe w bath and quite cage time . I try and create longer and longer habits of not screaming. If screaming starts after a certain amount of time being out of sight or not paying attention, I try to head that off by coming back in sight before screaming starts, giving them a little attention, and moving them to a different perch play area. Moving them around seems to reset brain . I also make sure to go up to them and do a lot of praising when quiet and say what a good quiet birdie.
What are yours scream triggers or times?