That saddle is WAY too narrow at the whither for your horse. Can you see how the points of the tree (that would be a few inches down from the metal stud you see right near the top of the pommel, understand?), are ON TOP of the hors's shoulder. They should be down , just behind the shoulder.
Do not use this saddle. Sorry, you are right
The one that "sits on the whithers" if it's due to being too wide, you might be able to pad it out, but only if it's too wiide all over.
She is fat, but really, really cute.
Have you thought about a treeless saddle?