I haven't watched the show since the middle of season 2, but I know that before I just quit that I was frustrated with how Elena was being written. All of the things I loved about her in season 1 (all of the things I liked about the show in general in season 1) were being overshadowed by her romance with Stefan; she wasn't a great friend anymore, she wasn't there for the people she loved. Katherine messing with the Stefan/Elena relationship made the writers push her character into being only about that relationship, and as someone who enjoys her relationships with her friends and family over any guy, it was making me not enjoy her character anymore.