A neoprene pad might help, but your best bet is to stick with a mounting block (or milk crate, or tree stump, ...) as much as you can until you can mount light. It can be pretty rough on the horse's back to mount from the ground alone all the time, even if you're an expert mounter, and if the process is pulling the saddle, imagine what it is doing to the horse's back and ribs.
Some of the pudgier horses will just have their saddles slip; just the nature of the beast. 14.2 and 1100 pounds sounds really heavy to me, my guys are about the same height, and 8-900 pounds; I think they look fat, but it really depends on body type, too. Low or mutton withers can really contribute to sideways saddle slippage, as well.
My sister rides the same saddle and runs poles, barrels, keyhole, and whatever else is offered that day. Does darn good for the equivalent of showjumping in a dressage saddle. If she doesn't ribbon, she's just outside that grouping. It can be done.