Agree, finding a biomechanics expert helps! My trainer works with Mary Wanless several times annually - so she gets some good training on an ongoing basis. She has a good eye (btw, she is available for clinics - p.m. me if you want info), and can often help find out what is causing position or effectiveness problems. Sometimes you are thinking, oh, I can’t keep my elbows to the side, and it turns out it is caused by riding with an arched back, or some such thing you would never have connected to your elbows.
BTW - I had an issue with my elbows sticking out, and part of it was figuring out a CONCRETE way for me to remember it. Telling me to lower my elbows didn’t work, telling me to keep my elbows at my side didn’t work. Telling me to touch my elbows to the top of my hips did work. For quite a while, I would ride with that mantra, elbows touch hips, until it became 2nd nature. That was just a personal thing - finding the right way to communicate the solution.
I will also agree, lunge lessons help. Many years ago, my prior trainer (also a biomechanics trained instructor) agreed to give me lunge lessons for months - nothing but lunge lessons every single week. It made a huge difference with my riding. The problem is - finding a HORSE who is good for lunge lessons, and an instructor who knows how to make them effective. But if you can find that combo, take advantage!
I have ridden with several “non biomechanics” trainers and clinicians over the years, and I always gravitate back to the biomechanics ones, I just find them that much better for my learning.
My current trainer (I actually just really started riding with her in the past year, although I’ve known her much longer, and she started horses for me) is really good at discussing theory too, and of manipulating my body so I can feel what she’s talking about. So, some lessons may involve 15 minutes of discussion and moving my body around - a mild rotation of the thigh, pulling on the leg, etc. These are very small manipulations, but make a huge difference in riding.
If you can find such a person, and they genuinely know what they are talking about, TAKE ADVANTAGE of the opportunity!