Cant remove upstairs hot water cartridge

[Saxon Elliott]

So the hot water in only my upstairs shower barely trickles hot water out. After a little research I found it to be just replacing a cartridge. When I got to removing the cartridge I hit a snag. Their was no locking clip only a locking nut/metal cover, that I took off but the cartridge wont move at all. How do I remove this cartridge? It is a double handle bath/shower in an older house

 

[SHEPLMBR]

That slotted brass piece around the plastic has to be unscrewed. Looks like a Moen. They make a tool for that.