worse scenario is you have to rebuild it, i rebuilt mine for around 200 bucks. it looks very complicated but if you follow the manual and diagrams its not too bad

 

I would suggest purchasing a case of Delron III, ATF and changing fluid at least 3 times, and driving for 5-10 mins between each change to mix old/new fluids. See how it drives after these changes. If still bad, you may want to try 2-3 changes of Honda ATF Z1 fluid (pricey at $8/qt).

A cheaper, but more difficult way to drain all old fluid is to pump the old fluid out as new fluid is introduced. Check youtube videos and search "atomatic transmission" for the video which describes the procedure. It is not difficult, but is not for everyone.

Suggest you also check resistance of the mainshaft speed sensor at the sensor and again at the ECU. If sensor resistance (400-800 ohms) is good at sensor and at the ECU, the ECU may be faulty.

good luck