When did this start to yellow? Looks like it got pretty big and was healthier at some point?
What have you been using and how much water is it getting?
If it was my plant, I'd make sure there's nothing wrong with the roots (too wet, fungus, etc), put it in fresh potting soil with a sprinkle of bone meal and see what happens. Water it the first time with a complete fertilizer solution at half strength - unless the soil you're using has ferts in it. And add a couple of spoonfuls of epsom to the water too.