Well, you are past the hard part, and also past the point of no return! My guess is that one of the side rubbers has come out of place. At this point, I think you grab that with some channellocks and twist. Need some common sense in the picture......I think the cartridge will self-destruct, which is what you want at this point. Depending on where it breaks, it will be easy or not so easy to dig out the remnants!. But if you apply enought torque that you sense the valve body connections are at risk....STOP.
You might try to squirt some 3 in one oil ( not WD) onto the body, and then work it back IN about an inch, then back and forth, to see if that helps.
Main thing to keep in mind as you do this, try to do nothing with METAL tools which will scar up the INSIDE of the body.