Your posture has the power to convey a variety of moods to your horse. I have found that I can pick a mood and use my body to communicate that mood to my horse. For a horse that is anxious and insecure, I can be easygoing. For another horse, I can be more strict and edgy, to make sure that they don't try to take advantage of my boundary system.
I've always just called this concept 'mood riding' and I use it all day every day. It goes along with my last post about becoming a different version of my trainer self depending on the needs of my horse. But the question I thought I'd throw out there quickly is, what do you think that a mood and posture of insecurity and a lack of confidence convey to your horse? Posture and mood don't distinguish between the rider and the horse; it is not possible to say to your horse with your body, "I am stressed about my inability, not yours buddy". So if you are insecure in yourself or lacking confidence in your ability to do something correctly, or worse yet, if you are mad at yourself for not being a good enough rider, imagine how your horse interprets this!
One of the most important lessons of my life was when I realized that I wasn't communicating "I love dressage. I love riding. This is fun. Thanks for playing along. You're the best." to my horse. I wasn't saying those things because I was so disappointed in my own riding skill at the time. So my body was saying, "Not good enough. You are failing. You will never get this." I thought I was only saying that to myself, but oh no! posture tells the whole story and horses are listening and internalizing that story.
Try switching gears. Try telling your horse through your posture, "We got this. This will be fun even though it's going to be tricky. We better stay focused. Let's have fun while we do this." I promise your horse will like this new story way better. After all, who doesn't want a happy ending?