Sickly Dumb Cane

I am relatively new to the botanical community and am in need of some troubleshooting for my Dumb Cane Dieffenbachia. I purchased it healthy and repotted into the container shown in the pictures below. It was doing well for a few weeks near a northwest facing window but because of low temps during the winter months I moved it away from the window, upstairs under a northwest facing skylight. I fertilized with general fertilizer and having been using a grow light for a few hours each night to promote growth. I also occasionally place it in the bathroom while I shower for some added humidity. It started out with a few yellowing, droopy leaves towards the bottom of the stem which I chalked up as natural process of growth so I removed those without much concern. This morning I woke to a very droopy, mushy, yellowing dumb cane and I am not sure what to do next. I love this plant and don't want it to die but am not sure what I'm doing wrong! Could it be too much humidity? Not enough drainage? Please help an amateur plant lover :(

 

I would say way too much care. For a while, I would remove the potted plant from the white pot, just put its own pot on a plate (I assume I am seeing a potted plant placed into a white jadiniere). Don't water it again until it feels a lot lighter. Remove any water from the plate after you water it. These plants grow well in the shade. Grow lights are not required. For a new plant, fertilizer is probably not required for at least several months, and during the winter when the light is low and it doesn't want to be growing, it shouldn't need fertilizer. I don't know about humidity, but it's probably not required. I consider this a plop in its corner and let it mind its own business kind of plant. Now it needs to recover from overwatering. Once it does that and you have figured out how much water is the right amount, you can put it back in its white jardinière, but lift the plant out after you water to make sure there is no water in the bottom of the white pot. The discoloured leaves will not recover. You can cut them off when they look bad, but keep some leaves on each stalk.

 

Oh, I just reread your "potted into the container shown" bit. Does that container have holes at the bottom? If not, the plant needs to go back to what it came in, or something with holes, with the whole thing light enough that you can lift it to feel when it feels light enough to need water, and a tray to collect excess water, so you can pour away the excess water.