Interesting pattern. I couldn't tell you exactly what the problem is, but if your plant is still hanging in there and you still don't know, here's what I would do:
-transplant it into a container with excellent drainage. Something with a good sized drain hole at the lowest part of the pot, elevated over the saucer to allow air flow: most rotting and wilting diseases are caused by fungi, and fungi tend to be pretty thirsty. You want to prevent constant moisture. Things like pots without drain holes or those decorative pots you put the other pot inside of are kind of dreadful because they just create a perfect environment for fungal diseases to thrive.
-when you transplant, sniff the soil and examine it for signs of mold. If any of the roots are necrotic (and some probably will be), amputate those.
-inoculate the new potting medium with mycorrhizae. If you can find a broad mix of fungi and bacteria that's good but if not I'd try trichoderma first, because it's a good beneficial fungus that helps with a variety of rots. You want your inoculant mixed in well all around the roots of the plant.
-pray to the gardening gods, I guess. Good luck and I hope this helps t.t
