Whose fault is it? 

In a nutshell… Yours! 
What is so ding-dang hard about taking the blame for something that didn’t go as you wanted with your horse? I really have a hard time understanding people’s resistance to this.
1) It didn’t go as you wanted. So, you either had too high expectations for the level of training that EITHER you or your horse have. That’s your fault for expecting more than you’re capable of delivering.
2)Your level of skill isn’t good enough. Again your deal. It doesn’t mean you’re a bad person. It just means that your level of skill isn’t good enough to accomplish that in that moment. Or… ever if you don’t learn from it. So what? And So What?
3)Your horse’s skill level or training wasn’t sufficient to accomplish it. Again, yours. Does the horse need to perform the skills YOU want for his horsenal needs? No. If his skill level isn’t up to the task, you haven’t gotten him there in whatever means you had available.
Try this for a while. It will become easier the more you practice (just like anything). When something isn’t going as planned, start your sentence in one of these ways: I should have… or, I shouldn’t have… Just give it a shot. Get your ego out of your own way.