I had a VERY bad experience with that issue. Unfortunately, I just do not know how "not to take things personally", doesn't matter what. Also unfortunately, this is the subject about which there is no much knowledge even in academic nursing. I even heard the opinion that cliques formation is somehow "inherent" to nursing as a profession (Cliques in the workplace: The good, the bad & the reality - CNN
IMH(umble)O, there are two types of units most prone to this evil. These are either cushy, frequently unionized, customer-service oriented places with overabundance of policies and regulations, or the opposite - the units where stress level is extremely high, support is zero and things are falling through the roof at any given moment. In either case, staff naturally clings to each other to survive - in the former case, because they do not want to leave their sweet and cushy jobs (or jobs they perceive as such) doesn't matter what, and in the latter one because they have to survive then and there.
Again, in my limited experience, one of the best remedies for this is not constant speeches about teams, working together, every patient being the center of the Universe for everyone, etc., but supporting (or disciplining) every nurse in the unit as a personality and an independent clinician. It is a way more trouble for administration, of course, than just being always nice with "old guard".