This is a great post and I've definitely lived the consequences of having a horse that was not at all suited to my abilities. When I was 9, I wanted to join my sister in the 4-H club. At the time, I had already been riding for 5 years, had taken lessons and was not half bad for someone my age. One of the parents in the club told my parents he had a 4 year old QH gelding that would be great for me. Totally bomb proof, never spooks, etc. My parents started into horses the same time I started riding and so they trusted this other person who had a ton more experience than they did.
Luckily, they only leased him instead of buying him for me because he was a disaster. Bucked me off almost every time I rode him, used to bite me, had no ground manners and many more things. If I were to get on him now, I would probably be a lot more successful, but at age 9, I just didn't have the weight or strength to teach him or be able to stay on. In hindsight, a 9 year old rider on a 4 year old green broke horse probably wasn't likely to succeed. (I know there are exceptions to this, I'm only meaning in general)
After returning him, my parents bought me a 12 year old Arab mare who was trained, had showed had been in 4-H and who just had more experience period, but was still "spunky" enough that she presented a challenge for me to grow.
I did learn two things from that experience: The first was tenacity - getting on time and time again, even if it was just to walk around the arena. The second - knowing when to quit and move on.