My girlfriend ALWAYS pes the blame....?

So whenever my girlfriend is wrong and we get into a scuffle about it, she always brings something up I've done, no matter how long ago it was, so she can take herself out of the limelight. I've told her no one is perfect and she continues to insist that she is (in a serious manner). What should I do?? I feel the relationship is in a strain and no matter how hard I work hard NOT to do anything wrong she always makes me feel like the bad guy. We've been together for 4 years and she has a ring on her finger.