Go back to a mohair or alpaca cinch with roller buckles. Classic Equine makes a really nice cinch. http://www.smithbrothers.com/27-strand-vintage-100%-alpaca-and-mohair-straight-cinch/p/X3-02105/
All leather latigo and off billet is very important so the horse can breathe. Make sure you do not have too thick a saddle pad making the saddle roll. Make sure your saddle fits. Pinching in the shoulders will make a horse seem girthy.
Make sure your saddle is not too far forward, a lot of English riders start out putting a western saddle too far toward. Make sure you have a hands with of space between the cinch and the inside of her elbow. The bars of the saddle should be behind her shoulder, not on top of it.
The comment that waiting for fittings until you get another saddle is kind of silly. Fittings can be transferred to a new saddle.
Lastly, don’t tighten your saddle to quickly. Snug it and walk her around. Do it in increments. Do not over tighten the cinch. Pretty easy to do with roller buckles. After being snugged up before mounting, you should be able to slip your hand under the cinch below the buckles. After you get on, walked around, check it again.
Good luck.