(ABC News) -- For Sandra Oh, the decision to leave "Grey's Anatomy" at the end of its upcoming 10th season did not come easy.
The 42-year-old actress told The Hollywood Reporter that she sought therapy to make the decision to move on and pursue other opportunities.
"I've gone through a lot of therapy over this," she told THR through tears. "I started thinking about it because I had to prepare myself. I gave myself two years to emotionally let go. At the end of last season, Shonda took me aside and said, 'What are your thoughts?' I said, 'I'm ready.'"
Oh has played prickly doctor Cristina Yang, best friend of central character Meredith Grey (Ellen Pompeo), since the ABC series premiered in 2005.
"Creatively, I really feel like I gave it my all, and I feel ready to let her go," an emotional Oh told THR. "It's such an interesting thing to play a character for so long and to actually get the sense that she wants to be let go as well. [Cristina] wants to be let go, and I am ready to let her go."
Although "Grey's" has lost other original stars, including Katherine Heigl and T.R. Knight, creator and showrunner Shonda Rimes said Oh's departure felt different.
"This year is going to be bittersweet for us. We're both going to savor every moment of Cristina Yang, and then we're going to give her the exit she deserves. And when Sandra walks out of my door, 'Grey's Anatomy' will once again be forever changed," Rimes said in a statement.
Rimes told THR that she hasn't yet begun scripting Yang's exit, which will be addressed at the end of the upcoming season, but the story "isn't about her leaving the show as much as it is Cristina Yang's growth as a person and as a surgeon. Cristina Yang is not going to magically decide she wants babies, no."
Oh told THR that she first began considering a wrap to her "Grey's" run in May 2012, when she and the other series' original stars signed two-year deals to take them through the 10th season. It's unknown whether the other original stars will remain beyond season 10, but Rimes told THR she hopes they will stay.
Oh told her co-stars about her upcoming exit at Tuesday's table read for the show's 200th episode.
"I seriously think I need that much support over processing it over this next year for me to be able to leave fully and leave in a way that I feel like Cristina deserves," she told THR.
"Letting go of such a huge part of my life … I want to cry right now," Oh said. "'Grey's' has been the most important role of my life. Playing Cristina and growing her and growing myself in her, I want to usher her into the next section of her life in the best way possible. It's got to be Cristina Yang's happily ever after."
When she does finally depart -- likely for a return to theater -- Oh said she'd like to take Cristina's stethoscope and ID badges as keepsakes.
"Grey's Anatomy" returns Sept. 26 with a two-hour premiere.