It might be helpful if you told us a bit more about your current riding, whether you used to canter easily, that is with good balance and in control of the speed and direction of the horse? And whether you have tried cantering again or not yet?
There is a correct aid or cue to ask a horse for canter - in fact you may be recommended more than one, but I learned to ride as an older adult and I never learned to use the correct aid properly because horses cantered for me very easily and always on the correct lead. This could be what happened to you in the past?
The most important thing in riding, our RI said, is to prepare the horse for what you are going to ask it to do. So it is what you do in trot (or walk) in the preparation for canter that really counts. It isnt just a question of going faster in trot because a horse cant run into canter. It has to bring its hind leg under its body, so you need more energy but at the same time to gather the horse back, (shorten the rein a little) and then release into canter putting on your leg, if needed.
You are right that people say the inside leg should be on the girth and the outside leg a little back. So if this worked for you in the past, do it.
But it can be a hindrance to newer riders trying to canter RS horses. Or maybe Jane with Ziggy. And it proved a hindrance to me out hacking when I was asking my horse for a particular lead cantering on a straight track. I think that the rider's mind may be on putting their outside leg back. This has two results - the rider is thinking back rather than forward at the moment when you want to create the onward flow of the horse forward into canter. And in older riders it may twist us in the saddle and create tension too. If you simply turn your head looking in the direction of the canter you want (which one normally does at the corner of a school) that has the effect of bringing the outside hip of the rider forward and leaves room for the outside hind leg of the horse to push forward into canter.
You should be able to feel whether or not the horse is primed and ready to canter. If it isnt, dont ask for the canter. Just go round again and wait till the horse feels ready to do it. That is, you have created the situation where canter comes easilly.
However, my first suggestion about returning to canter would be to find a good lunge teacher and ride your first canter on the lunge -