If the saddle fits you, then don't get a new one. If you take it to a saddle repair place, they can put a smaller tree in it. Its alot less expensive than a new saddle. You may be without a saddle for a few weeks, but its better than it not fitting your horse. Don't get a crupper, it serves the purpose, but can be quite uncomfortable for your horse.
My horse has high withers, and his saddle doesn't slip forward, it slips back.
There's one little trick you can try if the saddle does seem to fit pretty well... typically you attach the girth on the first and third gullet straps. However, you can attach on the first and second if the saddle tends to slip forward or the second and third if it tends to slip back. This is a minor adjustment, but it can tend to help at least a little.