Boat Momentary

i have a motor guide truster trolling motor when i bought it use it you could hook it to the battery and it would start running even in the off position using the handle and it wont ajust speeds ethier.just one speed.so i bought a big foot trolling motor switch.whitch has a momentary on/off.now i can hook it up to the battery and it wont turn on untill i hit the foot switch.whitch is good.but i still cant adjust speeds.only one speed.i tryed to find the part causing this problem,even called motor guide.my queston.. is how can i adjust speeds is there something i can buy to do this well besides a new trolling motor...