No one should risk their safety in order to canter. Canter is not compulsory. I havent seen the other thread, but I tink we sould look at the trot. But have you asked to canter? Does the teacher know you want to canter?
By posting to trot, I assume we mean USA posting, in UK rising trot. It is called posting because it was invented by post boys who until then had sat the trot.
I think that sitting the trot is a very important step in learning to ride. And one needs to sit the trot before one can ask for canter. But most people learn to rise to the trot first.
You can learn to post regularly by watching the outside front leg of the horse in trot. Rise when that leg goes forward and sit, then rise next time it goes forward. If you get out of time, just stay sitting for an extra bump and try again. Think of swinging your hips gently forwards between your hands.
To sit the trot smoothly you need to understand what the hind legs of the horse are doing. Riding in walk you should feel the rhythm of 4 beats and your seat should dip, first one side and then the other as the horse moves its back legs. Beginners think that their seat rises when the horse lifts its hind leg that side. But this is not so. When the horse takes its hind leg off the ground, that corner of the horse actually dips. Imagine a table. If you remove the supporting leg at one corner, that corner will dip.
Next time you ride, try to feel this rise and fall of your hips in walk. Once that is easy, see if you can concentrate and feel the same side to side movement in trot. It isnt easy but learning to sit trot safely is a very important stage in learning to canter.
Good luck to you.