Starting at 50, I noticed that my body didn't have the flexibility of youth. Some17 year old girl who started riding in the womb would say "Your leg needs to go here and your toes point over there" and I'd think, "You stupid b&%#&. If someone twisted my leg into that position, it would take 3 months in the hospital for me to recover!"
Maybe it was because I'm a guy, or because I spent 40 years jogging before getting on a horse, but they would tell me to relax into the saddle...and I would think, "This IS relaxed!" My hips and thighs had built in tension, good for keeping me healthy thru 40 years of jogging, but I was squeezing with my legs without meaning to. The tension in my tendons saw to that!
I also found a lot of mental imagery that I suppose works for some was awful to someone with an engineering background. "Your arms are hoses, pouring into the horse's mouth..." - YGBSM! My fingers don't leak squat! Or 'Your horse's energy flows from his hind legs into the bit, and back into your hands'...yeah! It it did, my hands would be ripped off my body. I needed instruction like "Force your knees as far apart as you can...you are still gripping with your knees...FURTHER APART!!!!"
A good lesson horse for starting would have done me wonders. Let's just say it isn't a good idea to buy a horse before you start riding, or to trust someone who says their horse is "Perfect for a beginner!" Sounds like you are already smarter than me...
When I finally got serious about lessons, I was lucky enough to take lessons from a woman whose emphasis was on how your balance affects your horse, and how your behavior can make your horse more willing, confident, etc. It can include little things. When you pick the feet of a good lesson horse, the horse will be sizing you up and deciding if you are someone to work with or not.
If you haven't read it yet, this thread gives a good mental framework for interacting with a horse: Every rider IS a trainer -- every time you interact with a horse