Can't say I share your enthusiasm for this plant, its close relative Mimosa pigra is a declared noxious weed here (and many other parts of the world) and difficult to kill off. Mimosa pudica is also a declared weed but being a small plant isn't noticed as much or get the same attention.

They usually grow in very wet conditions and like full sun. When M. pudica is older it copes better with being allowed to dry out a little, but as a seedling I think you're keeping it too dry. If you think "wet tropical", that's it's natural habitat.

This weed live in open area. Near streets or any places that get fullsun all day, far from big tree or tall buildings. It usually live among other weeds, which mean young pudica get some shade on early grow. They easy to find during dry season, when many taller weeds strugling and die, this pudica still alive. Farmer here working on paddy field using no shoes, so this thorny plant is dangerous and they will kill it when they found it. If you wanna grow it, consider grow it in full sun all day long. Less water is ok. Here in humid/rich water area, taller/bigger plant not giving room for this plant to get sunlight and grow, so open dry environment is where it commonly live.

Hi, GS. Is this a seedling someone dug for you from the landscape, or something that was purchased at retail in the pot it's currently in? If the former, it might be that the soil used is compacting and holding much too much water. If the later, its issues still might be related to over-watering, but the results wouldn't occur as quickly or be quite as dramatic as when the soil is completely inappropriate due to excessive water retention and its tendency to compact. I'd use a wooden dowel rod or skewer, sharpened on the ends and stuck deep into the soil as a 'tell'. If it comes out moist or stained dark by the soil, withhold water. If it comes out dry, it's time to water. Check twice each day until you get a feel for the intervals between watering.

If it's in topsoil or garden soil - anything that compacts easily, you really should repot as soon as the plant's health/vitality allows. Fertilizing can be addressed later, when you start to see it pushing new growth. Answers to the questions I opened with will provide us with more insight and a better sense of what remedial tack to take.

Al

Is this a seedling someone dug for you from the landscape, or something that was purchased at retail in the pot it's currently in?

I'm pretty sure it was purchased from a nursery-there's a barcode on the pot and a couple of words above the barcode which seem to signify it was purchased from a nursery.

I would have repotted it as soon as I could have, but since it's a seedling I was afraid that messing with its roots could set the plant back significantly.