Nina, who played Elena Gilbert in the US TV show, said being on the show was like being at college.
"It was almost like actor boot camp, or a college experience," Nina told The Hollywood Reporter. "I started college in real life, but I never finished.
"I went to high school in real life, and then went to Degrassi high school, and then four years as Elena - 12 years is a long time to go to school. I need to graduate! I'm ready now. I'm ready to go into the real world. I've literally grown up on this show. I was a baby when I started, and now I'm a woman."
News that Nina was leaving the hit TV series emerged last month.
Nina has played Elena since the show's inception in 2009, and she said she knew for a few years that she was going to leave, although news of her exit leaked earlier than planned.
She added: "It leaked a little earlier than we anticipated, which forced us to make a statement prematurely. But it was a blessing in disguise, in retrospect. It didn't hit me; I got to say goodbye with my family and with the fans."
Nina shared plenty of photos with fans documenting her last few days on set, and it proved to be very emotional, as she filmed her final scenes with co-stars Kat Graham, Candice Accola and Paul Wesley.
The Vampire Diaries series finale - which marks Elena's exit - airs this Thursday in the US. In the UK, TVD airs on ITV2.
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