Sally was one of the show's original characters, synonymous with Summer Bay, and Ritchie had played her since the show's pilot episode.
In 2008, at 28, Ritchie said it had become "really obvious" it was time to leave.
Watch: Kate Ritchie on Anh's Brush With Fame. Post continues below video.
"I didn't know where she ended and I began or vice versa, and I had to work out whether I was more than that," she said in an interview with Anh Do on Anh's Brush with Fame.
Ritchie explained how frightening it was to leave behind what had been her life of 20 years.
"What do I actually do when someone doesn't hand me a schedule on a Friday afternoon and tell me where to be on a Monday? And they were just the logistics that I knew I'd miss. I think it was the other stuff about missing the character and trying to figure out my identity without this other person, that was the stuff I hadn't really considered too much and that hit me quite hard," she said.
She fought back tears as she spoke of stepping away from her "best friend for 20 years".
"[Sally] gave me purpose and a sense of achievement. I feel differently now, after loads of therapy. I thought the only thing I'd ever done of value in my life was when I was her.
"I was lost, and I wondered how I was going to survive without her. How were people going to react to me if I wasn't Sally? Sally was a great person, a great friend and a great daughter... did people only feel something for me because they liked her?" she said.