Nothing Will Fit!

It's true! I absolutely cannot find a saddle that fits Ava. My BO lets me use their western saddles and none will leave an even sweat mark on her back. My english saddle may be the closest one to fit but if she spooks I don't want to go flying off. The western saddles always leave dry spots on each side of her withers - this horse has it almost in the exact same spots that Ava has [click here] but since she is white I can't tell if they are actually making sores, all I know is that they are making dry spots... Maybe western saddles just don't fit her as well??? Stupid question but it could be true. Ugh, I feel like she could be much calmer if I just found something that would fit, any suggestions?