Well, firstly, I have a lot of respect for people who are willing to change what they do not like. It takes a lot of courage. Also, your horse is beautiful.
I don't know what your price range is, but there's a brand called Rider's Choice, and they sell through ebay. They're made in Alabama, and they're exptremely high quality, and you can get a NICE custom fit saddle for around $500. You can call Jerry and talk to him PERSONALLY about fit and size, and he can make you whatever tree and whatever seat you need. They're extremely high quality for the price, they last forever, and are extremely comfortable.
But I agree, do go to a tack shop and sit in several different saddles and see what't he most comfortable. Remember that, because western saddles are measured from the pommel to the inner edge of the cantle, a 17" saddle with a deep seat (a steeper incline of the cantle) is going to be roomier than a flat-type seat.
You can also get all sorts of seat cusions for any saddle.
Also, maybe invest in a bareback pad until you find a saddle that fits you perfectly. Your saddle is going to affect how you learn - your seat, your balance, your leg placement, and your comfort. A bareback pad will improve your balance, perhaps balance you've been lacking due to the size of your saddle, and will also improve your seat.
I hope all of this helps! Best wishes. :]