Western help please!!

I ride English and I`m a fairly good and balanced rider with a strong leg position.
The second Im in a western saddle I become extremely unbalanced. If I try to sit the lope or jog, I have to hold onto the horn to keep me in the saddle, and often feel im being launched out of the seat. Ive shortened the stirrups as much as they will go. My leg does not feel secure at all although the stirrups are already to short for a western seat I`m told.

In order to be seated properly in a western saddle, you need the stirrups to be longer. I put mine at about the length I would have my english stirrups for dressage. You can't feel the horse under you as well in a bulky western saddle, but the basic idea is to push your butt into the seat, and ride with the motion, being nice and loose, not tensed up, or gripping with your leg. A friend once put it to me this way "you are supposed to be a sack of potatoes in the saddle". Hope that helps.

The saddle may be to big for you too. Here is a chart you might find helpful.
Western Saddle Seat Sizing Chart

according to the size chart the saddle fits. Am I supposed to push my legs forward to be this sack of potato...

Keep your legs back, knees should be barely bent. Heels down. Sit back and ride with the motion. At a canter, your bum should have a "scooping" motion. Hope that helps...

"Rock your hips like Elvis" is what a older trainer told me haha.

Your legs should not be pushed out forward. If you look down, you shouldn't see your toe was a good rule of thumb I was taught. You are not supposed to be a sack of potatoes. You will be looser than riding english, but still require to move with the horse and have proper position.

If you had a photo/video I would be able to see if you were doing anything majorly wrong.
Western feels a lot different than english, especially if you are riding an english horse with a bigger movement.

First step in riding Western is to have a Western trainer help you sit in the saddle. Second step is to sit in the saddle with "airplane arms." You need to learn to separate your arms and your seat/balance.

You need to sit on the padded part of your bottom for balance. Ride without reins for a while so that you get your balance. You should be able to teach your horse to turn when you look towards an object. I wouldn't go faster than a walk until you're very balanced in a Western saddle. Your knees should be barely bent. In fact, one way to tell they are OK length is to stand up in saddle. Your bottom may barely lift off saddle 'cause you practically are standing while you sit in Western saddle. It's good for my old knees.

A horn is NOT used for balance.

Try to feel your horse. Really FEEL him. Feel how his hips move. Let his hips pick yours up and move them. It's like you are dancing with him in the saddle and you have to follow his lead and move with him in order for it to work.

Some other tips are to sit deep in the saddle and to "sit on your pockets" which means to tuck your butt under a little and keep an open pelvis. So relax, and take a deep breath and melt down. Then lift yourself up slightly and you should be able to be more balanced.

Oh, and don't look down or at your horse's head, I've found that makes it harder to balance too. I hope that helps :)

You have gotten some great advice from previous posters so I will add my 2 cents on something not yet mentioned. Just give yourself some time, it takes a little while to adjust to the whole different feel and motion of a western saddle. Let the stirrups out a little, rock back more on your butt than on your pelvic bones, and rock your hips with the gait. Don't put all your weight on your feet, that will only make it harder to balance. Just enough to keep them in the stirrups is best.

And Joshie is right, a horn is not for balance. It is for roping or one of those "OH, CRAP!!" moments.