How to grow the tender sensitive Mimosa successfully
If you want to keep your sensitive Mimosa healthy long term I think are three key points to understand about it:
First - Mimosa pudica and the other members of the genus Mimosa are very hungry for sun and so are actually not good indoor plants. Most problems with them arise from not enough light. They are much happier outside, exposed to direct sun. So if you bring your Mimosa outside in open shade where it will get at first just a few hours of evening sun for a couple of days until it acclimatizes and then move it where it'll get full direct sun all day long she will feel much better. That's the ideal spot for the sensitive Mimosa during the warm months in temperate Europe. And it will solve 90% of your issues.
Try to give it maximum sun exposure during winter too. Even though it will be inside in a heated room it should be on a sunny windowsill.
Second - never, ever let the substrate dry out completely! Always keep it moist to wet, never dry. They do not rot from over watering, they rot from low light.
And third - they need rich, organic substrate and ample space for the roots that can support the rapid growth and mitigate the often inadequate mycorrhizal relationships that occur when growing in a pot. Good ingredients are peat, beeches leaf mould, some milled pine bark, some sieved good garden soil and some well composted plant matter plus coarse sand and perlite for drainage. Aim for a mix that is lightweight, porous and does not compact too much when watered. Water poured on it should drain quickly. If not growing in full sun use even sharper drainage. Use larger than usual, oversized pots.
In a small, confined space like a pot no matter how rich the substrate is initially it gets depleted over time. Some nutrients are used up, some are leached out and so sooner or later you will have to start fertilizing it. Use a good balanced fertilizer with micro nutrients / trace minerals. Apply it regularly according to the label.
So in a nutshell the ideal location is for example on the cacti table outside, but with ample watering. Think of your Mimosa as a resident of a sunny bog or a roadside ditch and you'll imagine what it likes the best.
The most common problem aside from the environmental conditions but related to them is the presence of spider mites which can quickly and almost covertly destroy your Mimosa. The best way to deal with them is by blasting the plant thoroughly and from all sides with a strong flat stream of water. The idea is that the spider mites are very small and tender creatures and that you use the mechanical force of the water to knock them off the plant. Even if they are not instantly killed they are either severely damaged or carried away and not able to get back to the plant. It is best to do it outside or on your balcony or you can do it in the bathroom too. Use a specialized garden hose nozzle that produces a flat stream. If needed you can support the plant with the palm of your hand or against a wall. Repeat the procedure for a week and you will forget about the pests, but it is good if you do it routinely from time to time. The plants like it, do not be stressed about the way they drop their leaves, they are actually very tough! Add some fertilizer and you will see your plant regrow and be happy again very soon!
On the photo is my Mimosa polycarpa happily flowering outside in Sofia, Bulgaria.