For certain cats, shelters do more harm than good, unfortunately. One of mine was a stray and we went through the formal process of taking her to the local shelter (pound) to confirm she didn't have an owner and get her checked out, etc., stating that we clearly wanted to adopt her if she had no owner. She went backward in the shelter (they flagged her as having "behavioural issues" - she initially started to warm to staff, but then became very withdrawn and defensive after they tried to wash her chin and stayed that way, apparently). She steadily recovered when we brought her home (no doubt being in a familiar place helped), although it took years for her to get used to strangers and her to stop responding to a couple of trauma triggers we were able to identify (I don't know what happened to her in her first few years of life).
To this day we're not sure whether they would have tried to adopt her out if we hadn't been so keen to take her. She has never been hostile towards us and her default is to run and hide if anything disturbs her (so on the rare occasion I get scratched if something startles her whilst she's on my lap or I'm holding her).
I wish you all the best with Maylin and hope that, with time, she'll improve.