I have some thoughts on this as I myself never mastered the correct aids for a canter transition. (I learned to ride as an older adult in my sixties) Was talking last week to an RI about this, as a couple of lesson horses cantered for me tho they apparently wouldnt for other students.
Sometimes lessons concentrate on what is correct for the rider. That tenses you up as you aspire to do well. And one forgets the horse.
I learned canter transtions somewhat differently - At my RS, (after canter on the lunge) we cantered from walk and we didnt really do it on the corner but a little way before the corner. When I learned trot to canter, again it was different as I learned out hackng on the straight. This may give you sme fresh ideas? But discussing last week why horses liked to canter for me, we concluded that it had nothing to do with the aids for canter. More the philosophy. To do anything in riding we are taught, you have to put the horse in a position where it is able to carry out what you want, Our RI meant physically able, but I include mentally able. Then you ask with a minimal cue. Then you do nothing, leave the horse free to carry out your order. It is the horse who needs to organise its legs, breathing etc.
So in a conventonal lesson or doing the aptitude w.t and c test at a RS on a horse I dont know, the important thing is to prepare the trot in advance, as you approach the corner. Just gee it up a bit, and use the reins to bunch the horse up a bit with its hind legs under it. i.e. not running trot. Then just before the corner you sit, but you sit without upsetting the active rhythm you have just created. The horse brain seems to recognise then that the combinatin of active trot, your sitting and the corner signifies canter is coming. All one needs to do is say Not yet, not yet, and then at the moment you want canter to think OK - sit back a little, relax your hands to let the horse move forward, relax your thighs to allow the horse forward between your legs and touch your heels lightly to the horse to signal the canter. Breathe slow and deep and think in time to the three time beat, if you need to relax.
Leave everything to the horse. I believe that the preparation tells the horse what to expect next. The horse knows I want canter. I often say that horses cnter for me because they know I like to canter. Now I have revised that. They canter because they understand and have got themselves ready for it.
If the horse does not canter,I never continue. I never ask again. I go through the same procedure, normal trot, prepare the trot and ask. I also dont go large but circle round to that same corner for a second "ask". And will possibly use stronger leg aid in the ask and say the word canter.
As one does this geed up trot, some horses dont get it. You already know there is no potential to canter. If that is the case, I dont bother to ask for canter. If I dont know the horse I may tell the RI that the horse has no canter and they may tell me a bit about that particular horse and how to ride it into canter. But some horses, which i do know - need something different. I mean to ride lots of walk trot transitions or to ride large in a vigorious trot to loosen the horse up, or collecting the trot and extending it. Or, if you want to bringthe hind legs under, some small circles in walk. For me it is better to have the horse listening and to make canter easier for the horse than to plough on round and round the school with many failed asks for the canter.
Apart from anything else, repeaed failed asks for canter destroy one's confidence as a rider. Whereas using these strategies reminds me, the horse and the RI that I am in charge and know what I am doing.
Being an adult student as you have found is a complicated matter. It shouldnt be increasing your fear. If it is this RI who is the problem, may be that is aother issue altogether. I know that the RI from whom I learned most was also one of the most useless when she had me in the school. I was for her by definition an elderly non starter - think RDA. That RI was a Carer.
Contrast the RI I went to when I dreamed of riding a flying change before I got too old to learn how! Pony and I learned together, miraculously moving up through dressage canter and when I eventually rode a flying change people righty admired her for that. There are teachers who predict you will amaze everyone including yourself. And provide the horses and tuition that makes it possible.
That is something for you to consider. I cant ride for an RI who lists all the things I did wrong. I need the freedom even in a lesson to be competent and adult with a horse.And then I flourish.