That’s odd…honestly, it may now be impossible to clean because the fleece has been ruined (it sounds like the girth has been through a lot, both in the process of getting dirty and your attempts to get it clean). I suggest just buying a new girth. You might try buying a leather one and getting a fleece cover to make it more comfortable for your pony. That way, if the fleece gets ruined again, at least you won’t have to buy a new girth. No matter what, though, if you buy real fleece, I suggest ONLY cleaning it with Melp: https://www.smartpakequine.com/pt/melp–250-ml-1862 . This stuff is amazing, and is made for real wool, sheepskin, washable leather, etc. Typical detergents will ruin real animal products, so stick with this cleaner and you shouldn’t have a cleaning problem again. I swear by it; it’s all I use on saddle pads, etc. (I suspect that your current girth is too far gone to save it with Melp, but you could try.) Good luck!
"Gentle in what you do; firm in how you do it." -Buck Brannaman