If you are talking about the horse you have in your barn thing-she's very pretty! I don't have tons of experience in saddle fitting but what I'd look for is a semi-QH bar tree OR if you can find it, I'd suggest trying an arab tree saddle on her because they are made for arabs with a typically higher wither, so they fit are supposed to fit it. Of corse, like all saddles, they don't always do what they say they do.
I know that a saddle pad can not correct a saddle fit, nor should it have to but Because your mare looks to have a slight sunken in wither, just to make her more comfortable in a western saddle, i'd try looking for a pad that has a little extra padding in the front. Try to stay away from cutback pads because if they are used incorrectly, they can be very uncomfortable for the horse, but something with a slight built-up padding in the front may be helpful.
If you have any friends that have western saddles in a Semi-QH bar or arab tree, I'd try asking them to see if you could try it on your horse to just see how it fits. Just know that not all brands have the same measurements so try to stay within that brand and around that maker year if you find one that fits.
good luck on your saddle search :]