Ok I'll just go by what you have said and won't try to read between the lines to see if you're competent enough etc.
But, what I would do with this sort of problem, and, funnily enough, I'm having this problem with one of the horses I am working for an owner at the moment. She has got a beautiful trot, but as soon as she goes to pick up canter, she panics, runs, gets unbalanced and falls on the forehand, is this.
BEND, BEND and more BEND!!! You want to do a lot of work in shoulder in, spiralling in/out of a 20m circle using leg yield (bent leg yield, not true, straight leg yield for this exercise).
Get an active, elastic, stretching trot on a 10m circle, or even a volte if your horse has the muscle and training to do so. When you are happy with the softness and engagement you have, start to leg yield out with the horse wrapped right around your inside leg. Ask for canter while you're still in leg yield, and through the canter maintain the bend, remaining on a 15m circle. Only ask for a few strides, then bring horse back to trot. Again, maintaing the bend through the transition so you don't lose the horse's shoulder and put it on the forehand.
This is tough work for a horse that is only starting it, so only do short periods of it through your ride. Just put in canters randomly, don't have a set 'now we will work on canter for 20mins' block. Go by how the horse is feeling, if you feel that you have an active, engaged, light and responsive trot that is bent around your inside leg, by all means ask for that canter. Don't make canter a big deal.
A take home message, just to drill it in - bend is the key!