Do you have any advice for beginners cantering fast horses ?

Lunge first so when you canter your horse will be more tired. Another option is a stronger bit to control more.
Half halts
Keep your shoulders up, and your seat light (ie. not bouncy/driving). I was on a quick mare when I was staring out, and it was hard for me to learn not to constantly pull on the mouth, because that tends to lead to a tug of war match.
Keep your but attached to the saddle (no half seat) half halt don't just yank on the reins
Don't tighten up you hips and pelvis, just really sit to the beat of the canter so you can remain balanced and feel more grounded in the saddle.
Sit deep in the saddle and relax
Sit tall, relax, keep your heels down, and don't pull.
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