I've read in many articles that you should not grip with your thighs, But how can I keep myself on a horse? I tried to be relaxed and stopped gripping with thighs , while catering, but when the horse pulled down his head, I felled off with my head to the ground! I'd appreciate it if you'd kindly let me know what should I do to hold my body with the horse?
Practice balance and stretching exercises, maybe even vaulting, while somebody, preferably, a good trainer, is lunging your horse. That way you can relax and forget about steering the horse for a while, letting you get comfortable in all the gaits.
I didn't mean how to have a comfortable seat. But I meant how to keep myself on the horse?If I don't grip, then how can I be attached to the horse for when the horse pulls its head and front of it' body down ?
As you develop your seat and balance, you also develop a stronger core and a seat that is independent. That helps to prevent gripping and unseating. Sometimes we just have to step back to the basics, in order to fix such problems.
It's a strong core that gives you balance and allows you to relax the rest fo your body to flow with the horses movements. The tighter you grip the more rigid you make your body and the harder it is to stay on. It's like asking "how do you stay on a bike without thinking about balancing all the time?" If you are stiff while riding a bike, you will fall off.
I'm not an instructor or an experienced rider, but here is what I've found...FWIW:
First, don't have your weight too far forward. When your center of gravity is in the same place as the horse's center of gravity, you make it easier for the horse to balance. However, you also are at greater risk if the horse does something unexpected. Be at least a little bit behind the horse's center of gravity.
Second, depending on your style of riding, you can get deeper into the seat = more pressure on your butt. A loose leg helps with this since squeezing with the legs tends to raise your rump out of the deepest part of the saddle. I sometimes have to make a conscious effort to push my knees apart and to relax my legs. Weight flowing past your knees and into your heels lowers your center of gravity. If you squeeze with your thighs or, even worse, do what I tend to do and squeeze with your knees, you create a pivot point. In a sudden stop, you then tend to pivot around your knees and forward - face forward to the ground.
Third, it is OK to have your feet a little forward. Heels under hips works well with a well-trained horse moving with lots of collection. That doesn't describe my world. If your feet are a little forward and your weight flows into your heels, then in a sudden stop your feet help to brace you against the sudden stop - but even then, you want the weight flowing into your heels without any interruption. I'm not any kind of expert, but I like my heels in line with my belt buckle. Sometimes even slighly forward of that, since my mare Mia is fond of "The OMG Crouch"!
Fourth, I like my legs to be wrapped around the horse because that seems to help with sideways hops. Wrapped around doesn't mean squeezing, just wrapped. Draped. I started riding at 50. Given how tight my hips are, I have to have my toes stick out some to get my legs draped around my horse without tension in my legs or squeezing. I accept that because a relaxed leg letting the weight to flow into my heels is better than a stiff leg trying to hold my toes more forward.
Fifth, because my horse likes The OMG Crouch and hopping sideways to see if I'm still awake, I like a secure saddle. It isn't cheating to use tack that helps you. I'm fond of Australian stock saddles:
Notice the Mickey Mouse ears in the front. Those poleys help a lot when things go wrong. If I get too far forward, or my horse hits the fan, or she spins suddenly or stops without any notice, the poleys keep my hips in place and aligned with the horse's back. I also like safety stirrups. In western saddles, a wide pommel or swell can do some of the same thing, but I like the poleys because they are lower on my thighs and shaped to match the angle of my leg.
Again, I'm a nobody in riding. These are things that have helped me as I started learning.
When you are gripping with your thighs, you are, as my father would have said, riding the saddle not the horse.
Work on your balance, and learn to move with the horse, by relaxing, and feeling it.
Also, you need to learn to read a horse better, as they telegraph what they are going to do. And cantering is when some of them will decide to put head down a lot of the time, as they feel you are relaxed, and will try you.
Learn to take up your reins faster, and to read the horse better so that you can keep horse from getting head down.