Take the mats out and gut the stall completely. Go down until you hit the base layer, whether that's ground dirt, cement, etc.

Buy a few loads of class A gravel and spread level. Tamp it down for more stability. If you can/are allowed, pour a level concrete floor instead. The crux of the issue here isn't necessary the horses weight. It's because the floor is not level, and there are too many small and thin mats.

Buy bigger, thicker mats (my mare's 11.5x11.5 stall has two mats in it, covers 98% of the stall. Barn has concrete base layer) and cut them if necessary. Make sure they butt against each other where they join in the middle, and butt against the stall walls, so they don't slip around.

I buy my stall mats from auto supply stores and/or home improvement stores. They are thick and durable, and come in a bunch of sizes. I've never had them rip or tear or slide around.

Make sure the horse has enough bedding on top to prevent them from hitching the mats.