Elena's life changes when she becomes a vampire.
One morning Elena woke up with Stefan and Damon by her side. When she saw them she asked in shock "what happened Stefan?" and Stefan then replied in a sad tone "Elena, you were in an accident and you had vampire blood in your system an now you're a vampire." Elena, on hearing that became very terrified with tears rolling down her cheeks.
Stefan, with his very loving self, didn't want Elena to become a vampire because he was aware of all the pain and guilt which she would have to face later on in life. He spent weeks upon weeks trying to teach her how to feed on animal blood so she won't hurt anyone. Despite all the training Elena could not have fed on animal blood because of a doppelganger curse. Stefan could have felt all the pain which she felt.
One day she went to a party with Damon, thinking he was the better one to train her to feed. They had a time of their lives feeding on people and then compelling them to forget everything afterwards. They danced together and party with each other for hours not caring about anything.
Later that night she went home and she was thinking of everything she and Damon did that night. She then realized that Damon was the better person for her because she was more like him.
After a few minutes Stefan came to visit her and he asked her "so how was your time with Damon, what happened?" Elena knew that she enjoyed herself, but she also knew that if she told Stefan she had a great time he would get jealous but still she didn't want to lie to him so she said "Stefan I know you went through a lot to help me, and I love you for that and you were always there for me when I needed you. Stefan I don't want to hurt you but I also don't want to lie to you. Stefan, I'm in love with Damon now, I know I said I'll always love you but I no longer feel that way... I'm sorry Stefan but I had to tell you, I couldn't keep lying to you anymore."
Tears invaded his eyes and flooded his soul.… He wasn't expecting these words from Elena; he was hoping to live a very long and happy life with her, one that would last for all eternity. He then spoke, and he said "Elena, please don't do this. I honestly don't know what I'll become without you" and Elena said "I'm sorry Stefan but I can't live a lie. You would be okay." On hearing this Stefan became speechless; he knew that Elena had made her decision, so he left without saying another word.
The next day Elena went to Damon's place, to tell him about what had happened with Stefan and her the night before. When they tried looking for Stefan, he was nowhere to be found, they even tried calling him but it was useless. They then realized that Stefan took this whole matter very seriously and he had left. Later that day Stefan left a message on Elena's phone "Elena I've gotten all your calls, but I had to leave, if you're happy with Damon then so be it, I'm not going to bother you." Elena was full of guilt knowing it was her fault and she went home wanting to be alone.