1988 Honda Prelude Tranny not shiftin

Transmission problem
1988 Honda Prelude 4 cyl Front Wheel Drive Automatic

i recently pulled my motor an tranny out to replace the rear main seal and when I dropped it back in an hooked every thing up and all that I took off to test drive it and transmission wont shift now. Its almost like its stuck in third or fourth gear an wont shift up or down I can manually shift it into second gear and itll take off little better but if I leave it in drive you can hear the motor trying really hard to get the car going however when I do finally get up tp bout 50 or 60 mph it runs like it should but wont downshift or any thing else what could be wrong with it?