First time here and first time home owner. I was in the process of replacing some old, corroded and broken shower fixtures. When trying to remove the shower control lever, which even after removing the screw holding the front on, it broke. I do not know a whole lot about plumbing, so I am unsure if this is fixable by me or a professional. THe stem coming out of the wall has flat spots around the edge that I assume I can put a wrench too. Will this allow me to remove the broken parts and replace? Is it just a matter of installing a new cartridge (I think that's what it is) or do I need to install a who new valve. attached are some images. [ATTACH]54727[/ATTACH] [ATTACH]54728[/ATTACH][ATTACH]54729[/ATTACH]

Bottom part of mixet valve stuck in pipe. - have tried CLR and Vinegar to loosen, no go - have destroyed old valve plastic cover, but retained screw - have tried using hair dryer to heat pipe, no go am I missing something here? I put the new valve upper stem back into old lower thats stuck in pipe and water back on, does not leak at least, wantto get that lower part out. any suggestions? also old upper stem had some sort of grease on it, is this necessary for new part as well? thanks[ATTACH]20707[/ATTACH]