It could be the solenoid valve, with the coil on it, for forward, is stuck on. Can you hear it click when shifting to forward? Does it sound the same as the reverse one? You could swap the forward and reverse solenoids and see if it has the same problem only in reverse. If nothing changes when you swap solenoids and it still wants to go forward, drain the trans fluid, start it and if it still goes forward, sorry to say that the forward trans clutch is stuck and tranny needs to come out but I think that forward solenoid has a problem.