It seems there's something missing? I mean, there's gotta be something holding the cartridge in place, right? You can't just push the thing in the hole
and expect it won't be pushed out when you apply water pressure, eh?. Usually there's some kind of screw-on retainer ring thingy - what's that brass-looking
thing at the top of the hole? Does it have threads? Did you use violence to remove the old cartridge? Maybe looking at the other, cold side valve would
give you some clues.