What's going on with my transmission?

Since last week, the transmission in my 2003 Ford Taurus SES
had been having a difficult time shifting gears or getting
above 30 mph without me revving the engine and wasting a lot
of gas. I check the fluid levels and it I'd bone dry, so I end up
putting 4 quarts in there only to be told that I overfilled it. Still
having shifting and speed issues, I resort to using
transmission fix in a bottle, which is supposed to be a quick
fix. I still can put my car into reverse and drive and the car
does move, but never gets past the first gear, which tells me
the transmission hasn't completely gone out. What else could
it be?