your saddle doesn't fit, get a saddle fitter. if the saddle is too narrow, it is pinching your horse and causing unnecessary pain. You will need a saddle fitter to help you. If it is too wide a half pad may get the job done, but you should still go to a saddle fitter as a half pad should only be a temporary fix. what's more important, your new saddle or your horse's comfort
I understand but you need to get it fitted by a saddle fitter Bc the saddle most likely doesn't fit and she/he could tell you to get a new saddle or what type of half pad to use that would make your sadddle properly fit
You could always try a shimable half pad or a riser pad. If your unsure of where you need the support and how much consult with a saddle fitter, trainer, chiropractic, etc. so they can help you get the best fit for your horse.