I'm glad you've realised how deeply attached your bird is to you.
Can you imagine how she felt when you abandoned her in a petshop? Suddenly, whatever security she had was totally gone and she was put in a shop with heaven knows what other animals, noise and bustle. People would have been passing by her cage and looking or poking at her, maybe banging against it. She would have been terrified!
Now, you've brought her home again, only she doesn't know for how long. She doesn't know what will happen next. She doesn't know whether tomorrow will dawn as usual or whether she'll be shunted away again. She's afraid and will be for a while since her security was so damaged. You'll need to work hard to gain her trust back again, I think.
All you can do is to spend time with her every day. Be gentle and kind and don't allow any loud noises or sudden changes to upset her.
All parrots scream, just as all dogs bark and all ducks quack. At least the Alexandrine call is not so loud. You should hear my cockatoo when she's in good voice! She puts the Alexes to shame! My Beaks call for about half an hour every morning and for a little longer of an evening. It's part of their ritual, so I don't try to change it. I'm lucky, though, as my neighbours are far enough away not to mind the birds' noise. If it were a real problem, I'd be sure to work out the times when my birds were noisiest and have something planned to divert them during those times. For example, if your bird screams at 7am every morning, maybe you could choose that time to feed her? Or train her? Or give her something interesting to chew? Or teach her to make a different (quieter) sound like saying 'Hello'. It may not work, but it's worth a try. Punishing the bird won't stop its natural need to call. Covering the cage, shouting or squirting it with water will only terrify it and quickly lose whatever trust it has in you.
From what you said in your first post, your bird sounded like a good companion who was gradually learning to fit in with you. If you stick with her and make her your project, there's no reason to think she won't be an excellent companion to you for years to come. Why don't you check out some of our forums (eg. trick training or target training)? I've found that training my birds has given them something to think about and occupies their minds. Target training is especially good because most birds learn it very quickly and it's a very positive way of interacting: you ask something of the bird, she does it for you and earns a reward. Everybody's happy!
Do stay in touch! If you have more questions, please ask them! Most of us have had our own problems and we have a lot of experience to share.
