Saddle balance is as important as saddle fit to your riding position and communication with the horse. Do not assume because you saddle tree fits it is also balanced.
I recently went through a tough time fitting my horse, Mercury because he is what's called "uphill" in his conformation. Meaning his withers and back slope uphill from his hind end. He is very nicely built for natural collection, balance and dressage. This is one of the reasons I bought him. It has proved to be a fitting challenge though as my saddles fit in the tree size but all sit "down hill" on him. Meaning the back or cantle of my saddles sit low compared to the front making me feel as though I am sliding backwards. Now I am fighting for my balance against my equipment. To any rider this feels horrible! So imagine how much uneccessary muscle strain you are using consciously or unconsciously because of this. Now if you are straining, you are communicating tension to the horse as well as concentrating on balancing instead of riding and communicating with your horse. You are also possibly unbalancing your horse as well unintentionally. This can feel like the horses gaits are choppy and fast when he is out of balance. Also, our imbalance can cause a horse to be tight in the back and hold his head up in tension to compensate.
I find that western saddles often sit "down hill" on horses. Meaning the front of the saddle is low compared to the back. One reason is a lot of Quarter Horses are built and bred that way. The other is poor fitting saddles. Now you may say " my saddle looks fine it's higher in the front, but your not looking in the right place. You need to look at the spot where you sit. See if it looks level from the ground and more importantly "feels" level when you ride. Sometimes you have to sit in a few different saddles on a few different horses to note the differences in balance good or bad.
So now your thinking how can I fix it if it's off? Well IF your saddle is sitting high in front or back you have to check on a couple things first. If it is low in front it can be too wide of a tree for you horse which can be remedied by using a pad that is designed thicker toward the front then the back. These are easily purchased in western and english styles.
If you saddle is sitting high in front, you probably have too narrow of a tree which is not good. This can pinch and restrict movement, bending, willingness to go forward, cause soreness and muscle atrophy in the area. DO NOT ADD PADS the small trees! You are taking more space away and can create even more pressure points. You need to get a tree that fits.
If your tree fits and is sitting low in back (like mine), you can get a pad that is built up in the back and not in the front. My saddle I had specially made with the back built thicker in the panel so I didn't have to use a pad. I don't like using extra pads. Unfortunately, western saddles don't have any padding you can add into the saddle so you are at the mercy of just plain good fit, custom fit, or specially designed pads. If you are unsure of your eye and feel ask a qualified fitter in your area for help.