I've got a 12 y.o. integrated Grohmix shower thermostat (I think). The flow control lever no longer works. I started to disassemble it (I've seen the "Integrated Grohmix clean out" posting) but I got stuck trying to remove the temperature control ring. I know it can't be hard, but I just can't figure out how to do it. I've attached photos of the whole unit and of where I'm up to. I would really appreciate help.