They're not so different, when it comes down to it. Katherine was selfish and reckless and manipulative, and Elena thinks that sometimes she's exactly the same way when it comes to the Salvatore brothers.
She's selfish enough to stay with Stefan even when she knows Damon feels something for her. She even thinks she feels something for him too, sometimes. But she loves Stefan, and she claims it'll always be Stefan—just like Katherine.
She's reckless enough to let Stefan use her blood to learn how to control the cravings. She's reckless when she tries to save them all by sacrificing herself. Katherine was reckless enough to play the town of Mystic Falls twice, reckless enough to flaunt being a vampire.
Elena will deny it to everyone but herself, but sometimes she's manipulative enough to use Stefan and Damon further her own ends. She does it with Damon when she tricks him into giving her the book on Katherine's family history. She tries not to lead Damon on when she knows how they both feel about each other but something in her can't stop. She can't help the covert glances and smiles, can't help speaking to him as if her opinion she matter simply because it's her. And she does it with Stefan every time she kisses him, every time she says she loves him, because she uses that love to cover whatever it may be she feels for Damon.
And one night, she goes down to the tomb and admits this to Katherine. Spills everything. How she loves Stefan but is drawn to Damon. How she makes impulsive decisions and doesn't care about the consequences. How she uses both Stefan's and Damon's feelings for her when it suits her needs.
Katherine just smiles at her in that all knowing way. She sees the similarities too, and understands why Elena's coming to her for advice. "You can only control both for so long, Elena," she says, drinking some of the blood that Elena brought. "Eventually they'll start to get ideas and you'll have to choose. I ran away, bought myself some time, but in the end, I had to make a decision."
"Who did you choose?" Elena asks.
"Myself."
Elena hands Katherine another bottle of blood before closing the tomb. As she walks back to her house, she wonders if it would be so horrible to make the same choice.