My horse is 18.3 so also a very strong girl. When I start to canter she is very calm but when I stop she just goes to a very fast trot and never walks. How should I make her stop?

Try doing very small circles to make her chase her butt
I would first of all make sure your tack fits properly and that she's not physically uncomfortable for some reason when transitioning down. Then use half-halts and instead of transitioning to a trot (which she likely expects) try to go from canter to walk or halt. Then later reintroduce canter to trot transition.
One rain stop
Try circles and maybe even set up trot poles and go after them after your transition. Just a thought hope that helped
Maybe use a tougher bit. And remember to sit deep in the saddle and use words like easy or woahh.
try half halting a lot and if she doesn't listen maybe try circling and half halting
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