UNfortunately, the piece you need in order to "easily" remove it, is what broke off. I am not sure how YOU are going to extract the remainder, but do NOT do anything that will damage the brass body or you will be replacing the entire valve. I have some speciallized tools to solve a situation like that, but you do not and it would be cheaper to hire a "good" plumber than buy them. At least your cartridge is intact. The last one I had to remove the customer had already destroyed it before calling me which made it even more difficult than normal.