mysterious betta sickness

Question
A while back, a got a female betta sorority. All the other girls have been fine, but soon after i got them, i noticed one would stay hidden in the shadows and usually had clamped fins and pale colour. But she would come out to eat and had no other physical symptoms. I hoped she would get better by keeping up very good water conditions.
But when she stopped eating and i found her stuck to the filter intake on day, i took her out and started treating her with furan, which i've heard and read is very gentle and a very wide-range healer.
Now, this is shocking, but it has been well over a month since then, maybe two. She has still not eaten! She has darkened to an unnatural dark brown colour, and her tail is so clamped it's a small point like a needle. She floats at the top, sometimes on her side, and can't seem to swim down. Her swim bladder is not bloated. She's very lethargic and just lays there among fake plants all day and night and doesn't move except to breath from the surface. I've added a parasite cure and recently sulfa antibiotics (all 3 meds can be used together) and change the water every couple of days. There is no change, but a few days ago, she started breathing faster, her gills going in and out.
I feel so bad that she's lived this long! I thought she would've either gotten better or else died by now! I've thought about euthanising her, but if she's lasted this long, wouldn't it mean she could get better?
There is no other external symptoms that i can see.
What else can i do?
Please help me help her. I feel so bad for the little girl!

Answer
Hi Joleen,
 Unfortunately you have already tried everything I would have suggested so I really don't have anything new for you to try. Sorry.  

-- Ron C.
  rcoleman@cichlidresearch.com
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