Do a lot of transitions from trot to canter, keep contact (working onto the bit sometimes helps) and sit deep in the saddle so your pony knows you want to move forward to canter and not a faster trot :)
Get a nice forward (working) trot going. Make sure your not chasing your pony into the canter or else your canters quality won't be as good. Try sharpening up transitions by practicing them a lot. You can also carry a crop and use the 'leg, click, stick' method. Basically ask with you leg, if no response click, and again if no response use the crop behind your leg. As soon as you get any reaction reward your pony!! Keep practicing till your pony understands your leg aid and only needs the click or stick some times!!
You could try first working on trot/walk transitions to really get your horse listening to you and make sure they have impulsion from their hind end then it's just a matter of practice, it takes time for your horse to learn how to go around properly, be patient :)
I have the same problem with my horse. I use a special leg kommand to get her in canter but you can also train on not letting the horse trott fast before it canters. That was one of my many problems with my horse at trot to canter transistions. But ofcourse the horse habe to have the strength to go from slow trot to canter. I would also recommend using rewards. If you have treats or something you can reward it when it transition to canter or just a pat on the shoulder or a good voice.
My old horse used to be like this, if you get someone on the ground to help you and start from asking in trot, use very light canter aids and say the word canter, at this time get the person on the ground to use a lunge whip to encourage your horse to move forward. Eventually your horse should learn that the canter aid means to go straight into canter and won't hesitate!! Good luck! ☺️
Have a stick or a whip and use the ask, tell and now method. Ask them to go forward or transition into what ever pace, tell them that your going forward by applying more pressure and making your movements faster and if that doesn't work give them a little tap with the stick or whip. After two or three times of this by the time you even think or ask your horse to move forward, he's gone.
I recommend squeezing more (I don’t recommend kicking unless completely necessary) and clicking with your tounge. If your horse still won’t then just try holding a crop and when they act up lightly tap them and squeeze while clicking:)
The better the trot (or walk) , the better the transition. Reach a good contact, make your horse sitting on the hindlegs with inside leg-outside rein effect: you should feel his energy, like he is ready to go forward every stride, just waiting for you to ask. Then use a corner, bend him a little bit to the inside, keep you shoulder back and ask him to canter.
With young horses is easier to go into the canter from a fast trot, but as soon as they are balanced, you need to keep the horse steady during the transition, without running. For this reason I prefer the walk-canter transition more than the trot-canter one. In both cases, you need to prepare the horse for it, without asking for it out of the blue.
Another way to do a good walk/trot- canter transition is to start from the haunches-in: this exercise put your horse in the perfect position to start cantering.