I think I know what is wrong. The curvature to the spine is almost a dead giveaway. It probably has fish TB. That causes fish to lose their appetites, fade, and get hollow bellies and humped backs. It is especially prevalent in live-bearers. I had a dwarf gourami die from that. It didn't have any curvature, or hollow belly, which is hard to see in them, anyway, with their long dorsal and anal fins, but it gradually faded and lost color, and finally quit eating. When it stopped eating, and started to lose weight, I realized it had TB. I humanely destroyed it.
Start over with a new Betta and a clean tank.