Love and Hate such a fine line

There's always a time where you think about the past. There's always a time where you regret some things and want to change the past. Stefan is in that phase. He can't stop thinking about Elena and Katherine. Elena is with Damon his brother, even though he doesn't love her anymore he can't get over his relationship he had with Elena.
One day Katherine pops up and Stefan changes to a completely different person.