What is the best way to teach a horse to slow down his canter? My horse is way to forward and I don't feel in control when cantering as he just takes off.

try lunging him and working on vocal cues
I like to go on a biiiiiig circle (at least 20m, but could to be bigger if you have a wider arena) and spiral into a smaller circle. The bend helps control the speed and helps the rider sit and follow the horse. Just make sure you have enough weight in your outside stirrup as leaning into the circle will only cause the horse to go off balance and speed up more. Once you've maintained the speed on the smaller (~10m circle), gradually ease them into the original big circle. If the speed increases, spiral down again.
takes a lot of muscle to do canter transitions. do lots of downward transitions and circles. you are probably getting nervous and bracing as well, so you need to sit deep in the saddle and relax. do lots of softening exercises and work on collection
try doing a lot of downward transitions in the canter.
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