i would say the saddle is much too narrow for your horse. mutton withers can be such a pain to fit! my old 14.2 hand heinz 57 didn't have much for withers either, and let me tell you, if that girth was so much as one notch not tight enough, it was instant 'slide down the side' when it came to mounting.
i'd try to find a wider saddle. check kijiji out, i am often finding decent english saddles on there that fit a wide range of horses. everything from wide wide draft saddles to narrow tree 15 to 16 inch seat youth jumping saddles.