How do you determine when it's time to water? It looks like the result of excess water in the soil for extended periods; and, a high level of dissolved solids (salts) in the soil might be a contributing factor. When your soil is too wet, it limits your plant's ability to take up water, which results in a drought response. The first thing most plants do when threatened with drought is start shedding leaves to conserve water, even if the roots are drowning in a sea of plenty. Too, high salt levels in the soil work in combination with an excess of water to further limit water uptake. Enough salt in the soil can actually reverse the movement of water into cells and actually pull water out of cells (reverse osmosis) in exactly the same way that curing salt pulls moisture from meats like ham/ bacon/ jerky, ..... When you water, you should water thoroughly, so at least 15-20% of the volume of water required to fully wet the soul exits through the drain hole. Then, the plant should never sit in the effluent (water that has moved through the soil and out the drain). That practice provides a pathway back into the soil for all the salts that were flushed from the soil when you watered. In passing, it should be mentioned that root congestion causes loss of older leaves, as does decreasing light levels, but given the rotted appearance of the leaf stems, I'm thinking your issue is related to poor root health. You might lift the plant from the pot and inspect the roots to see what condition they're in, and start using a 5/16" wooden dowel rod (any hardware store), sharpened in a pencil sharpener to test the soil's moisture content. Stick it deep into the pot. If it comes out wet or stained dark from the soil, withhold water until it comes out almost completely dry. There will still be sufficient available water at that point to satisfy your plant's need for moisture. I think you'll find the info below to be helpful. Growing Overview Sick or Dying Plants? Al

If you can get/keep the soil, light levels, and temperature in the sweet spot zone, and do some other regular root maintenance every 2 years or so, you should be able to keep most houseplants in a high state of vitality for decades or generations. The reason a large fraction of growers have come to look at houseplants as 'disposable' is, they are expecting their plants to cheerfully deal with conditions at or beyond the limits they're genetically programmed to tolerate. You might find this helpful. It's a basic overview that will allow you to avoid almost ALL the pitfalls you are likely to encounter early on your path toward a green thumb. When you think of how much water your plant needs/wants in the soil, think 'damp', or 'moist' - like a sponge that's been thoroughly wrung out. Soils that remain wet or soggy for extended periods make the job of keeping plants healthy and attractive much more difficult and very often frustrating. When I took up bonsai as a hobby (before the net was a thing), I failed miserably because understanding how soils work and how water behaves in soils is a critical aspect of that endeavor. While it may not be as absolutely critical when growing plants in larger/deeper pots, it's still crucially important, something you'll eventually find out if your interest doesn't wane. Learning to recognize high quality soils, or make your own, is probably the largest step forward you can make on your container-growing journey. I know that, if I had someone to tell me that when I first started growing things in containers, I would have arrived at the threshold of being able to keep healthy almost anything I tried to grow in pots 3-4 years earlier than I did. Hopefully you'll read the overview and it will leave you with questions you want answers to. That's the MO that moves you ahead farthest and fastest. Trial and error is a painfully slow way to learn, because in most cases, you need some measure of stored knowledge to even be sure what you think you're seeing is actually what is occurring. Al

That is one wild old plant!! The shriveled stems may be dead. It may be time to start over with tip cuttings.

That isn't watermelon peperomia. That's peperomia frost. They actually like a lot less light. Mine is on the second shelf of a plant shelf and is about 18" from a grow light. It does very well and retains all its frosty color there. I sometimes ignore it for too long and it will wilt. Then I water thoroughly and let it sit in a saucer of water for 20 minutes. Then I dump out the remaining water and let it sit in the sink for a bit to make sure the pot has drained off before placing it back in its home. I suspect yours is either getting too much light or possibly too much water. My suggestion would be to cut off all the ugly leaves. Also cut off all the inflorescenses because they drain energy from the plant. Then put it in lower light. Let it dry completely before watering again. You might want to also try to pull it out of the pot to see how the roots look. Hope this helps.