When we first begin to ride, our toolkit (both literally and figuratively) is rather small. You may have two to three brushes and a hoofpick in your caddy. You may only know how to do 20m circles and diagonal lines at the walk and trot. You get my drift. The more experienced we become, the more stuff we accumulate and the more we know. Like many things in life, becoming a better rider calls for simplification and efficiency. When something you are attempting in your session with your horse is not working, modify it. Just like when you are working out, if an exercise becomes too difficult, lower the impact. There is no sense in forcing yourself to do it over and over again if it's not going right, causing pain, and deflating your morale.
Often when an exercise is too difficult for a horse it can be pared down to these few possibilities: lack of impulsion, stiffness, or being out of balance (ie. heavy on the forehand or a large discrepancy of ease between left and right). When confronted with difficulty, you'll need to choose an appropriate tool from your toolkit prioritizing calm, forward and straight.
For example, an issue that popped up yesterday was a rider could not turn her horse to the right. When she would try, the horse would grab the bit and rush forward. She was stuck in the "I can't turn" rather than looking into her toolkit to ask "What else can I use to turn?" My thought process: the horse is moving fine aside from when it's time to turn and then he loses his calm. Priority - keep calm and forward then worry about proper flexion. We have been working on counter-bending as a suppling tool. Since the horse had no problem turning left I told her to counter-bend him (to the left) and to circle to the right. The horse left the wall and did a volte to the right. Then I instructed her to stay in the circle and gradually ask the horse to bend to the right. His reaction was to slow down, but eventually they both succeeded in moving forward in the correct bend. The horse's main message here? In this case the horse was being more bossy than he was trying to communicate a stiffness on the left side of his body as I had seen the rider warming up with relative ease. The key was to use another tactic to arrive at the same intention.
Look to what you already know how to do with what you have. Challenging your creativity will develop new tools. Break down your problem like a math equation and solve the simplest things first: calm, forward, then straight.