I feel like this problem is way more simple than I've been making it, I'm willing to admit I've hit a wall.
My mare has been having this major issue with staying on the fenceline. I started working with her on it at the walk and had a lot of success, I managed to get her to really solidly understand by just going around and I'd let her take a step or two off it so that she'd know exactly what I was correcting, then pull her back to the fenceline. She picked up on it quickly now she rarely steps off of it and will bring her nose into each of the corners. Great!
But at the trot she continued to randomly pick spots (for some reason, the same general areas every time, which I distinctly recall no bad thing has ever happened to her in, she'd never spooked there or anything like that, nothing unusual about these spots at all) where she'd just make a sudden sharp turn. So I worked with her the same way, and now at a trot she will stay on the fenceline, rarely straying, and if she does it's quick and easy to correct and gets better all the time. Wonderful!
But then we get up to the lope. At the lope it's the toughest, because she is very athletic (and I am still working on getting back in shape ) and will just randomly dart off straight to the side. I've been very careful to be absolutely sure I am not putting any pressure whatsoever on the outside of her, but she just goes. I try to work on it the same way as I did at the walk and trot of course, but she isn't having it. I'll put my inside leg and rein on her, she ignores it. So I pull on the outside rein, she keeps on going where she wants with her head bent to the fence and eventually can't keep up moving like that and breaks awkwardly to a trot. Ew!
It's generally easy to control her at a lope. She'll turn in neat little circles, properly bent, on a soft neck rein....that's the part that gets me, lol. She's obviously electing that her little shortcuts are the appropriate way to navigate an arena.
I feel like a dunce for not being able to sort out this basic problem. I just know there is a simple answer that's somehow evading me...so what would y'all suggest?