"I mean I ride horses 5-7 days a week now, have been riding for 10+ years, and I can't explain to somebody how to canter?!
Homework assignment: Write 3 paragraphs on how, exactly, you walk across a room. Discuss how you walk straight, turn, avoid someone's shoes, and not fall down. Discuss how you maintain your balance at all times.
Riding a horse and walking across a room are done at the subconscious level. You cannot think fast enough nor receive the inputs you need with your conscious mind. At best, your conscious mind can help with the gross outer movements...but your body needs to learn the muscle movements at the subconscious level. If you started riding as an old person, like I did, you will soon find you can recognize faults in what you & others do, but cannot make your body stop doing them. As with many physical skills, all you can often do is try not to screw up too badly while hoping your body someday catches on and starts doing it right.
An instructor can help with setting priorities. For example, IMHO, an instructor can help by encouraging a student to keep a relaxed leg, and then worry about the position of heels & toes later on because a stiff leg will cause more problems than toes that look like a bird's wings. An instructor can also build a series or progressive movements, which is why most would start with a walk, then trot, then canter and then gallop. It is why some use a lunge line to work on a good seat before progressing to some other step.
Thus teaching becomes an entirely different level than riding, just as breaking a young horse is quite different than riding an experienced school horse. For some, it can become a passion that gives purpose and joy to their riding and time with horses and people. However, teaching riding well is as much a skill and art as riding well.
Even at the rational level: I'm volunteering tutoring math. Doing math is one thing (for me, at least). Explaining it in a way a young student can understand it is an entirely different beast! But when something clicks, it can be very satisfying.
BTW - you might also try showing her or recommending some videos. My favorite on cantering is this western one: