As a person who doesn't like to re-do anything and doesn't like to make mistakes, learning to accept an occasional "Ok, that didn't work..." was a difficult task. Having to let go of the fear of failure, the fear of messing up my horse, the fear of looking like a fool and a certain amount of perfectionism was like having to cut off my arm.
I still harbor those thought,s but now they skitter around the edges of my consciousness rather than clobbering their way to the forefront. It takes a little bit of wanting to do things right in order to succeed, but we have to able to accept mistakes and be willing to experiment. The only rule is that above all else, we try not to get hurt and try not to hurt our horses.
We could read every book ever written, take thousands of hours of lessons, watch zillions of videos and we will still come across that horse that makes us go "Huh, what is that all about?". Every horse is different and has it's own personality and quirks. There is no set-in-stone formula that is going to work for every horse. There isn't even one that will work for every Thoroughbred, or Quarter Horse or Arabian or anything else. There is no one way to work with a mare or a stallion or a gelding. Every yearling is different. Every 3 year old is different. And so on. Even when working with the same horse day after day, there are days when that horse will give you a different challenge. Our job is to be willing to experiment and to accept defeat.
We don't have to win every battle. There's nothing wrong with changing the subject or re-directing an exercise, as long as it will end up being positive and it's on your terms. If something isn't going right, there is no need to drill it until it does go right. What usually happens is both horse and rider end up exhausted and cranky.
If you are trying desperately to get a leg yield to the left and it's just not happening, examine why, first. Are you sitting too far right? Are you restricting with too much rein? Is your horse sore? Has he lost interest in responding to your aids because he just did 48 leg yields to the left and that ought to be quite enough? Rather than battle on, there is nothing wrong with taking a time out and walking on long reins for a minute. Or you could go and do something that you know both of you can do well. Do a canter circle. Do a turn on the forehand. It doesn't matter what, as long as it has a positive outcome. Most of the time, taking a break, or doing something different is enough to break up the tension and redevelop harmony so that the previously frustrating exercise becomes more productive.
When working on something new, the only way to figure it out is going to be to try it. You can not learn to do half-pass by reading a book. It may help, but you've got to be up there, applying aids and gauging your horse's reaction to know if you've been successful. Doing it wrong, or sloppy is not necessarily a bad thing, as long as you take note of what needs to improve and work on that the next time. It's akin to a child learning to write. There has to be practice. There has to be a bit of going outside the lines and being crooked. Certainly no one expects a small child to write her name perfectly the first time. No one expects you or your horse to do every transition perfectly the first time or every time. Mistakes happen.
Mistakes are okay, they are part of the learning process. I tell my students that a mistake is not a mistake unless you repeat it. Otherwise it's just practice. If you are supposed to end your trot lengthening at K, and you continuously let your horse drift left and miss the letter completely, then you are making a mistake and doing yourself and your horse a disservice. If you miss it once and notice that you were slack with your outside rein, then you can ride it again and make the correction. Of course, then you will finish your diagonal with counter flexion because you had too much outside rein. But it wasn't a mistake! Because it was a NEW problem. There was an effort made. Congratulations. Now, go try again.
In my dance lessons, I have put pressure on myself worrying about making a mistake during an upcoming performance. My teacher has told me. "You are going to make mistakes. Just accept it and go on. Everyone makes mistakes." Good advice, even if it is from someone who doesn't know a hoof pick from a Hanoverian.
The amazing thing about horses is that they not only allow you to make mistakes, but they forgive you for them too. Unless, they get hurt. Even then, there are horses who will put up with the pain and discomfort and continue to try. They shouldn't have to, but most of them will.
So make a mistake. Recognize it, give your horse a pat and then correct it. Mistakes are all part of the learning process. Those that don't learn from their mistakes are the only ones making them. The rest of us are just practicing.