I think you need a bigger pot. NBJ is very hard to kill, they need big pot & big root system to support their growth, especially in full sun.

If you think you have over-watered the plant, you need to remove the soggy soil and replant in new slightly damp soil.

Sounds like you have got a fungal infection in the root system.
Drenching with a fungaside may help. Condies crystals are good.
'Potasium Permanganate' [from the Drugstore]Mix about 1/8 teaspoon in 1 litre water. Or copper oxycloride is good too.
It will probably loose its leaves now, but they should come back. They are pretty tough.Good luck. L

More suggestions, if it hasn't died by now.

If there's a big crown of leaves, I would prune it back. Some people believe that defoliating it completely encourages it to grow roots and reduces moisture loss through the leaves. I'm too chicken to do that so I would cut it back to a third of its former size.

Take the plant out of the pot and VERY CAREFULLY AND GENTLY remove as much soil as possible without hurting the roots.

Soak the whole plant, roots and everything, in a, tepid, dilute solution with vitamin B for just an hour. (I think there are already mixed solutions call root starter. Read the labels in the garden stores.)

Then re-pot in a soil mix that is very, very loose -- lots of perlite. Water thoroughly.

Move the plant into filtered light or light shade, no direct sun to let it re-establish. When the stems and leaves are nice and green and turgid again, you can move it into more light.

DON'T WATER IT SO MUCH. Once a week of deep watering is more than enough.

If you tend to over water, use a auto-waterer. The self-watering pots I've tried don't work well for me.

Two gadgets are first, a rigid plastic 'tube' with holes along the sides that attaches to a bottle of water. You insert the tube into the soil. It will dispense water as the soil dries. I'm not promoting anything, just think these gadgets work well. Here're links to one place online that sells this:
http://www.gardeners.com/Search/search.asp Q=watering&Page=3&searchType=Product

Another watering gadget that is somewhat more attractive, at least smaller, is a ceramic stake with a tube to a water reservoir.

Here is a link that might be useful: ceramic stakes to auto water