Oh duh, I missed that part about it sliding forward. In that case I highly recommend the sheepskin pad, but only if you plan to use it directly on the horse and not over a baby pad. The sheepskin will not slip and should go a long way to keeping the saddle in place. I am notorious for going out trail riding with a very loose girth, and even after cantering up and down hills, my saddle stays in place. This is the kind of pad I use, only in black. White gets too dirty.
You just have to see your distance...you don't have to like it.