I agree with Sue, although not necessarily about tossing it. A large pot is very difficult to water properly because the excess soil absorbs lots of water and retains it for a long time. I suspect that the roots of your plant have been slowly suffocating and the effects are now starting to show up. I don't recommend doing it, but if you were to unpot the plant, I think you would find a very small root system that is struggling to survive.
I do suggest that you remove all the loose soil on the surface that is not in direct contact with roots. This excess soil is keeping the soil in the root zone from drying out frequently enough. That excess loose soil is also misleading you as to the soil moisture around the roots. Once you remove the excess surface soil, you can then water as soon as the surface of the remaining soil is almost dry. Add just enough water so that it reaches that same level of dryness again in about a week. Experiment a bit to determine the right amount.
It does need protection from direct sun falling on its leaves, but it otherwise it should be as close to a bright window as possible. If it is more than 4 or 5 feet from a window, it is not getting enough light.
It needs just water and no other supplements.