Damon and Stefan were convinced that you were Katherine, and they weren't willing to hear any other explanations. They thought your appearance was a disguise, and they were sure that you were trying to trick them. But you weren't Katherine, you were Elena, and you didn't know how to prove it to them. Damon and Stefan wouldn't listen to reason, and they refused to believe you when you told them that you were Elena. It seemed like an impossible situation.