It will come over time. I had the same problem with my 3! Do a lot of circles too, they help alot.
I like to do trot to canter first and then before I go straight into walk to canter ill do a turn on the fourhand and ask my ottb to canter out of it. It really loosens him up and makes the transition easier on him. Then I praise do it a couple times each direction and the start to do walk to canter transitions without the turn.
Try maybe to work on transition from walk to trot and then to canter.
These exercises are "heating up" horses, so if yours has a tendancy to get nervous, they shouldn't be done too many times at once. A few transition during every session is enough, to improve calm into transitions and come step by step to your objective.
It takes time but it's rewarding, promise !