Callard's character Liz McDonald, the Rover's Return landlady, was last seen in Weatherfield in April 2011.
Speaking to The Mirror, the actress opened up about her struggle with her late mother, who lost her long battle with Alzheimer's in January.
She explained: "Being ill and wanting to spend time with Mum was the real reason I had to quit Corrie last spring.
"[Alzheimer]'s is the cruellest of illnesses. It takes people and robs them of everything that made them them. By the time they are finally released from it and die there is nothing left.
"It pushed me into bankruptcy and I guess I'll be paying for it for years, but regrets? Not one. It was horrific. I've worked non-stop since I was 16 and somehow it all seemed it had been for nothing.
"Corrie schedules are full-on. It's all or nothing and I had to face a choice between the Street or Mum. I love everything about Corrie but there could only be one answer."
The 55-year-old has previously admitted that she enjoyed the ITV soap more when it aired three episodes per week back in the 1990s.
Photo gallery - The life and times of Liz McDonald:
Copyright: Rex Features ITV