My horse hates the saddle?

my horse has always had a real problem with her saddle, we bought her a while ago where she had an quite an ill fiiting racing saddle. we have had her back and teeth checked which are all okay, and have recently bought a made to measure saddle which fits her perfectly. but she's still wary of it and whenever i put it on she'll have her ears back and kcik out and try to bite. does anyone know why and of anything i can do to prevent this from happening?