I have a female betta for about a month. When I first got her I had her in with a guppy who starved herself to death too
but that was back when the tank wasn't fully cycled so I pushed it off as my mistake. She was in a 10 gal filtered and heated. The ONLY thing she takes interest in is cooked pea I have to wiggle it and make her chase it just to get her to snap at it, then she spits it back out. Anything else like blood worms, or mosquito larvae, ANY type of pellet... she won't even take a second glance at no mater how much I wiggle it about. She seems perfectly fine aside from her emaciated look (no clamped fins, no curving of spine, no scale loss... or anything like that) and is very active.
Recently I removed her and put her in a gallon bowel since it's much easier to keep track of her and clean in that. I do 100% water daily now. I hope it isn't TB because I have a lot of scratches and cuts on my hands and came in contact with her water everyday... but I really don't think it is... she is showing no other sings of illness besides refusing all food. Like I said this has been going on for a month and I spend an hour each day on her and it is very frustrating. I am very close just to give up on her entirely.