riding/trotting mishap

We were riding outside today and my instructor told me to do posting trot.. no problem right? Wrong.
He starts out fast and it only gets worse because she said I was leaning too far forward, he thought I was telling him to canter. So he takes off, and he's just running blind, wont stop, and I can't get him under control, im losing my seat and I can't pull on the reins because he runs from the pressure. And im trying to stop him, but im too scared. I was afraid he wasnt going to stop. I was terrified. This has happened before but we were inside.

I guess I know what I did wrong but I don't know how to prevent it
And I don't know how to stop the horse if he takes off again.

Im still really scared.

If it were me I'd sit back and get the horse going in a circle and slowly spiral into a pretty tiny circle. He can't keep running in a small circle, unless he wants to fall straight over, which no horse want to do.

Good luck!

Wallaby nailed it. Learn how to disengage your horse's hind end. One rein stop, shoulder-in, pulley rein, anything that involves putting the horse in a not-straight position. If his butt's on a different track than his front end, he can't run off with you. It is the single biggest confidence builder I ever learned.

What the others said. If he runs from pressure, or leans into the bit, he can't do that if you only pull one reign. Depending on how fast he`s running though, don't turn him too sharply, he could actually fall.
This may sound silly, but did you try to tell him to whoa. You have to say it like you mean it though.
Also, I know I`ve had a horse run away on me and my mistake was I had a death grip on him with my legs. I know it`s scary, but the more tense you are, the less likely you`ll get the horse under control. Instead of thinking about falling or trying to stay on just tell yourself `im in charge, im in control`
Ask your instructor to practise your emergency dismount and emergency stop (aka pulley reign). If you have those down solid, you`ll feel more confident should this situation arise again!

First thing. Don't panic, this will only make the situation worse. Perhaps you were tense when you were posting and were accidently egging him on.
Best of luck.

I think you've been given great advice so far. You absolutely can not go wrong with an emergency dismount - if all else fails. Turning in a circle is also a good way to slow down a wayward horse.

Definitely work on his cues for stopping.

Well said also if your going to far forward your leg is probably swinging back hitting your heels into his side. Just relax!

Is this your horse or a lesson horse?

All of the advice given has been great and you should definitely try it. However, if you are still scared, I would not get back on this horse for a couple of lessons. Normally I would say don't let the horse get the best of you but in this case I think you should hop on another horse a couple of times and regain your confidence before attempting to work with this horse again.

When my young horse starts acting up and I get nervous, I end the lesson on a good note. Then I ride one of the barn owners lesson horses just so I can show myself that I CAN ride the way I need to and boost my confidence. It helps a ton in terms of being brave enough to get back up there and show the horse who's boss.

Good luck and let us know how it goes!!

If this is a lesson horse your trainer needs to do some work with this horse or rethink who she puts on the horse. Doesn't sound like a solid horse for a green rider.

Either way, your horse or not, I agree that the trainer should be giving you instruction on a bend and stop procedure.

So often in a lesson there is only time to work on going forward, but a good instructor will also spend time on stopping.

I agree with Solo and G and K's Mom. This horse does not seem like a good match for you. A horse for a new rider (and I am one, so I should know) should be more stop then go. I would rather have to work harder on keeping my horse going then worrying about him running off with me cause I placed my leg wrong. You should really be asking your instructor (assuming its not your own horse) to put you on a different horse. If it is your horse, you should think about using a lesson horse meant for a beginner.
Leaning forward does not mean canter. A trained horse should only canter if they get the correct aids. I find if I start losing my seat at any point a push my feet way forward, which pushes my butt into the seat. This alone typically works then I can get back to my basic seat. If it doesn't work for you then you can turn the horse. Turn him into the inside, most horses will want to go to the center anyhow. Keep turning him and he will slow down and eventually stop.
Work on your seat and trying to keep your legs where they should be. I think you may have put your leg too far back and this could have put him into a canter.
Either way, this horse does not seem fit for a beginner, so you really should have a different mount while you learn.