One thing you can do that really works with most horses is to start on the ground away from the mounting block. The idea is to teach the horse to turn their neck and head towards you while you mount. It sounds odd, but I have been successful with it in the past.
1. As I said start on the ground. Have your reins over your horses neck as if you would when you are getting on any other way. If you get on from the left, take up the left rein, and ask your horse to bend his head towards you. Hold it there for a second, ask him to stand. When he stands for a second, give him big praise and release. Repeat this over and over, TONS of praise.
2. When he'll stand like a gentleman with his head bent for up to 30 seconds not moving, try the same action, and add your left foot barely in the stirrup. Don't worry about getting on yet, and if your horse is tall, feel free to lower the stirrup so it is comfortable for you to have your foot in there from the ground. Do the same as in step one, but ask him to stand still with his head bent towards you with your foot in the saddle for up to 30 seconds. This can get tricky, because the horse that evades mounting will try and step away. The rein pulled to your body helps keep your horse from walking forward. The trick is to keep your foot in the stirrup a little bit even if he takes a few steps. This involves some hopping and maneuvering on your part, but eventually he'll stop moving if you stay persistent. Practice this until your horse will stand still for you to put your foot in the stirrup and apply pressure. Remember, tons of praise. Give him some release from the rein pressure in between tries so his neck doesn't get tired.
3. Now that you have a horse that stands like a gentleman for you to put your foot in, you should be able to easily throw your other leg over without your horse running off. If you are comfortable, your next step can be to simply stand up in the left stirrup, putting your body weight in the stirrup but not fully over his back. Now you want him to stand still while mounting until you ask him to walk.
4. The last step is to take it to the mounting block. Put the block in a corner, so you have the horse between the wall and the block. The only place he'll be able to go is forward, and at this point you should have him trained to stand still for mounting. Practice the same things with him, bending his neck, asking him to stand quietly, etc.
It will take a lot of practice and repetition, but this does work. It helps keep the horses focus on you, instead of off in the distance. This is also one instance I'd suggest using a treat if you are comfortable. When you get to the point of mounting off of the mounting block, your horse will automatically turn his head for you to get on. From here, when you hop on and ask him to stand still while you get adjusted, you can offer him one single sugar cube as another form of reward. Don't overdo it, but it can help. The sugar dissolves quickly and won't interfere with the bit in his mouth.
Stay confident, tell him he's a good boy. This is absolutely something you can do on your own even if you aren't the most experienced rider ever. Good luck!
Owner and head trainer of SE-Wisconsin Horse Care
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