I am not an expert in plants, but do have some experience.
I would try getting it some soil that has some micchorzia (spell check needed). They are microrganisms that live in the soil and attack to the roots of the plants, feeding from the plants and in turn helping the plants absorb the nutrients that are needed.
Soil that has good nutrients in it as well is probably a good idea. Try looking up what mint plants need nutrient wise, and go off of that. You can probably find some common brand soils at places like walmart or home depot that would be acceptable.
After that, I would suggest checking the ph of the soil. If your going to purchase soil to use, this should not be a problem. The soil that it has been in for a while though could have its pH thrown off due to watering with acidic/alkyline water. Over time this throws off soil pH, effecting how well plants can absorb nutrients. I would try to make sure that the water you use is at a neutral pH to not effect the soil too much.
Other then this, the only other thing I can think of right now would be sunlight. If it is not getting enough sun light, it can not produce the energy it needs to survive. Check out online how much sunlight a mint plant should be getting, and adjust if necessary. Maybe it should be put outside if its not to cold.
Good luck with caring for this plant
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