1996 Opel Tigra

I Drove my vehicle very slowly through a large pool of water, thereafter, the sporty program sensor light came on and would not go off. Furthermore, my transmission (automatic) became very sluggish. I then continue to drive my car by changing gears, 1, 2, 3 and drive. I did this for a few months. The fluid was not changed in a while and I have not had a service either. Bottom line, my transmission has started slipping and now it will only work for a few minutes and when I turn the car off, it works good, but only for a short time. Is my transmission finish? It it an electronic problem? My mechanic tell me I need to change my transmission, I need a competent person to advise me!