Check you kickdown cable
Check the kickdown cable is adjusted properly, if the car still kicks into the next gear nice and hard then the transmission is still probably fairly serviceable.
To adjust the kick down cable pop the hood and locate your Throttle body, this should have two cables going to it. The one furthest to the rear of the car is the throttle cable, the one closest to the front of the car is the kickdown cable, follow the kickdown cable from the throttle body to the plate that holds the kickdown cable adjuster, it has a button on the front off it and a sliding nose part that faces the throttlebody. Press the button and push the nose part back in (so the nose gets shorter). Let the button go. Now hop in the car and slowly push your foot down on the accelerator untill it hits the floor (car doesnt need to be on, in fact DONT TURN CAR ON).
You should probably here some clicks as the adjuster (nose part) relocates, hop out have a look the nose should have popped out a bit. Close bonnet go for cruise.... Hopefully all is good.