What do I do about my one-sided horse?

Hello! There can be several causes for this, and you need to work through them all. The very first thing is to make sure the horse is well in its body. A vet check is a good idea, to see that all is as it should be. Once you have done that, here are a few other suggestions.

1️⃣ The horse's natural crookedness. This is about straightening work, where you focus on helping the horse find its balance. Identify which shoulder the horse wants to push out, as it will probably bend its neck automatically the other way. From your description, it sounds as though the difficult rein is the one with the dominant shoulder. Ride plenty of lateral work, focusing on bending the horse, the neck included, and move the shoulder along the bent line. The horse should move away from your leg until you feel it lighten in your hand on the same side. Take care that it is not you holding against the horse, but rather the horse lightening in your hand as it places its shoulders correctly. Work mostly on this rein. It may be that the rein you experience as the easy one is actually the difficult one, so make sure the horse has not simply over-bent its neck instead of rounding around your leg.

2️⃣ The horse's understanding of the aids. This should perhaps really come first, but you can work on both in parallel. Make sure your horse genuinely understands your aids, that is, that it responds lightly to your weight, leg and rein aids. Always check this at the start of a session, and put extra focus wherever you notice the response is not good.

3️⃣ The saddle. Does the saddle suit both you and the horse? If it is an older saddle, it may need an overhaul and possibly reflocking. If all of that is fine, do get someone out who knows both saddle and rider. For example, someone who has trained as a VBR.

4️⃣ Then we come naturally to the last piece of the puzzle. You, as the one communicating with the horse. Hand on heart, do you train your own straightness, stability and coordination? We ask the horse to become straight, but we cannot always sit straight over the horse in every situation, on a circle least of all. Ask someone to film you and the horse from outside the circle, and look at the sequences where you can see the pair of you straight from behind. Does your spine line up with the horse's? Are the stirrups level in every gait?

All of the above assumes, of course, that your horse is sound and otherwise healthy. If you notice the saddle sliding to one side when you look at the photos from behind, that can be a sign of developing lameness and, as said, worth checking, unless the saddle's flocking has simply become uneven.

I hope this gives you a few clues to work with!