I didn't know with the first horse, and some people assumed every behavior issue was due to my rider error - and I'm POSITIVE I contributed. What I was afraid of was that it WAS all me, and that I would not succeed with ANY horse - and in fact that I would RUIN a good horse.
What I found out though (and what I think should encourage other beginners) is that although an experienced rider can deal with just about anything, a new rider simply can't. A horse that has a great foundation of soft training... plus has a trusting, willing temperament... will be forgiving of mistakes as long as the rider does their part to be fair and 'listen'. It was such a relief to figure this out.
I do have to 'ride' this horse - she will occasionally want to discuss something - but it never ramps up to nasty, so I'm able to stay firm but calm. These discussions are few and far between now.
I have found that 98% of the time, when something goes wrong, it's handler/rider error. That's ASSUMING a good match and a handler/rider who knows what's what. When you have a beginner and a BTDT horse, it's not unusual for the horse to get savvy and say, "Oh I got your number kid!" and to take advantage and bully the human. That just happened with my DH. Found a mare who was BTDT, been through everything including mounted patrol training, and SHOULD have been a good one for DH to learn the ropes on. Well, she tried on the trainer and lost. Tried me on and lost. She went ok for both of us, so we put DH on her. Yeah, well, let's just say she dialed his # so quick, it was probably the shortest test ride in history. She made it SO clear that she's done with beginners. Nothing ugly, nothing nasty, but very firm. So was DH to blame? Only for being a beginner.
He's riding my Cloney, who is a little hot for him but very kind hearted and watches out for him. I've decided to hide my eyes and just get on every week or 2 and fix anything that comes undone. He'll make mistakes. So will Cloney, because Cloney has never had to teach a beginner before. But I can trust that horse with my life and DH's life, he would NEVER do anything to hurt either one of us on purpose. DH may take a little longer to learn some things because he can't just get on and think about what HE needs to do, he has to think about keeping the horse on the same page with him, so that may slow him down a bit. They'll both learn from it. Nobody is going to "ruin" a perfectly good horse. I think they'll both be better for the experience.
DH still has to learn about 'firm'. He's good on calm, a lot calmer as a beginner than I ever was, even as a kid. The horse is really very calm, but is a typical Arabian and more reactive than I'd pick for a first ride. Somehow, like riding in the back of a pickup truck, or riding a bike with no helmet or running the streets all day and all night like we did as kids, I think they're going to both survive. In the meantime, I'm working on making his 2nd horse.
Your first horse taught you a lot, even if you didn't like most of it at the time. She wasn't a good fit and that may have been her most important lesson to teach you. Now, you KNOW a fit when you find one. You 2 are doing a real good job!