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.