Have you ever heard your horse tell you that something is impossible? You're working on a new move, adding some collection, asking for a bit more/crisper/sharper and your horse clearly says, "This is so impossible and no horse has ever done this and there's no way I could ever do this so let's just stop, okay?"
What if I told you that most of the time when we hear horses say that something is impossible, they are actually telling us that something is hard? They say hard, and we hear impossible, and then we give up and go onto something else.
When I hear the horse making some rumblings about something edging towards impossible, I ask myself these questions:
do they understand what I'm asking given their level of training and knowledge? E.g. if I'm asking the horse to do a downward transition, is that a task they're familiar with?
are they fit and strong enough to do what I'm asking? E.g. if I'm asking the horse to hold a collected canter for several minutes and they're only just learning the collected canter, three minutes might be too long to stay in the gait initially.
am I asking correctly and clearly? E.g. am I using the correct aids, or has something gone awry with my position?
If the answers to those questions is "yes", then all systems go. These questions help us figure out if our expectations are reasonable. For example, if a horse is slow to move off my leg, and they're ten years old and have been trained to move forward off my leg since they were four and I am asking correctly and they are not too tired to do it - then they sure as heck better move off my leg!
I can acknowledge that something is hard, but ask for the horse to try anyways. This is productive, athletically-induced discomfort, the same discomfort I feel when I am doing squats or holding a plank or something else that's hard. It may also be mental discomfort, like when I'm trying to wrap my brain around a new skill. Growth and learning is uncomfortable, but doable, and often even fun!
It is hard work to use one's body well. It is usually easier to do something else. It is easier for me to sit on the couch than go for a bike ride, but that doesn't mean that a bike ride is impossible. Consider your expectations and whether they are reasonable given the information you have. And then - go do that hard thing! In little, manageable (and likely imperfect!) chunks. Your horse will never magically become more collected/more through/more awesome without some input and guidance from you. We have to build that up, one piece at a time, being bold through our mistakes, messiness, and the difficulty of it all. When you were a wee one and learning to walk, it was really hard and messy and you made mistakes all the time, but more than likely you learned how to do it eventually.
Hard is not impossible; it's just hard.