Ahhhhh!!!
I would suggest that if you’re going to show, regularly ride up the centerline so your horse starts to learn that that is “a line” and he has to go straight and forward on it. It becomes a familiar track for him and he gets used no not having a wall for support. Start to choose in your mind where you want to halt and practice halting at that spot. Halt early, halt late - shake it up so your horse learns to listen to you and gains the strength and confidence to (eventually) halt straight. You can place parallel poles on the ground in various places on the center line and halt within them. Make them spaced “generously” at first and make them more narrow when your horse “gets” the exercise and then improves. And you can be sure you’re riding the halt straight. The visual cues can really help. You can do this on the beginning-ish or end-ish of the diagonal (where your horse can be straight going into and out of the halt). Good luck!
If the horse is not straight and the rider is not straight, then the halt will be crooked. The closer to the wall/rail, the more “support” the horse has to stay straight. The square halt tests the straightness of the horse and rider, which is its purpose. It’s not a stand alone trick (which is more how it’s taught in showmanship or halter classes).
Last summer I was riding a dressage school master. I found that if I engaged my core riding down center line or anywhere, I got a square halt. And if my tummy muscles were slack, the horse dumped on the forehand and fell off the track with her feet all over the place. That was a real wake-up call about keeping my core engaged all the time on every horse, except maybe after hour 2 of a back country trail ride :).
Anyhow, you might not get such clear feedback on a green horse, but still give him the best chance of success by making sure you are sitting in and have your ribcage lifted.
@J-Lu love the idea of the poles. We have a new coach at the barn who is very keen on visuals, I never knew what I was missing before! My long time coach hated them, wanted us to not rely on crutches of any sort, great when it comes to gadgets, but not so handy when it comes to learning, with poles, cones and barrels to help you ‘see’ what you are trying to achieve.
It’s a crutch for you and your horse. Your horse is not likely to step outside of the poles so teaches him to be straight. It is a crutch ideally for a short time but then you and you horse learn the aids and you can produce the same halt without the rails because you both “get it”. “Crutches” are OK if they help you and/or your horse figure out how to master the task at hand but you’re ultimately not dependent on it. Riding on the rail is a “crutch”. but not many people will admit that! Good luck with your horse! Be creative!
One other thing that might help, is to think of riding the horse such that you are telling him where he should go next, not where he should have been in the previous moment.
Very sound advice, and very guilty of not signalling early enough, but working on it.
Had an outside coach in yesterday, and got some more great tips. Best advice, stop trying to halt on centre line, practice a couple of halts on the rail, no more. Do far more exercises turning on to centre line. Then circling 10m off it and returning to straight, get him looking for the centre.
Best truck of the day counting the walk steps, 1-2-3-4, then when ready to halt go 1-2-3 and a half…it worked beautifully, because I rode into it.
OP - I read all the other comments (but skimmed a few) so please forgive if someone has already mentioned this. The reason you had trouble on the centerline may not be you necessarily. It’s a young horse. He stops on the rail just fine because it’s there to “hold him up.” This is a fairly common issue. You have to first teach him to stay straight off the rail. He needs to be able to balance without the visual aid of the wall (or fence or whatever). If you are riding outside, then I am full of baloney and you may scroll on past, LOL.
@blue heron - my personal mnemonic for the centerline letters is (I don’t count A) Dressage Legend Xenophon Is Great