I have an Aussie saddle that fits my TWH quite well, but I bought it new and sent in a wither tracing. I really like it. Don't give up on Aussie saddles, they're great for trail riding. I think you probably need one like mine with the serge panels since your horse has narrow withers. You can get info on this on Wither tracing and fitment instructions for Aussie Saddles
. There's a section on the bottom of the page on serge panel vs. fleece panels.
I believe you can get the saddle adjusted to fit a narrower horse, but I'm not sure how or who would do that. They also have quite a few saddlepads to fix fitment issues on that site. Of course if those options would cost a lot, you might be better off buying a reasonably priced new one and reselling the used one. I have the Kimberly Literider - I checked out the reviews on www.horsetackreview.com
before buying it because it isn't one of the pricier ones, kinda midrange.
I originally bought a used one, but my horse hated it, it was actually too narrow (she's kinda medium). I returned that, bought the new one and she's great with it. I'm much more comfy riding in that than I am in a western saddle and I feel more secure (I'm not an overly experienced rider).
The customer service people are great at Down Under Saddle Supply. I called them a bunch with questions and you get one free adjustment of the saddle if needed during the first year. I swear they aren't paying me to say this stuff (LOL).