The Brookside and Royle Family star speaks openly about her battle with bulimia in her new autobiography Things I Couldn't Tell My Mother.
"Back then my eating demon was out of control. I was constantly obsessed with food to the exclusion of everything else, and this went hand in hand with an intense self-loathing," she recalls in the book, which is being serialised in The Mirror.
"I began to eat any old rubbish, shoving it down my neck and then throwing it up again. I was treating myself like a dustbin. I'd cry with shame but I couldn't stop. It was an appalling time and I have been dreading this coming out because I haven't told my son or Goddaughter yet. I nearly didn't write about it - but that felt dishonest.
"The truth is, I'm ashamed I allowed myself to get so low over food when there are people in the world starving. Having played a psychologist in Waking the Dead I know what my character Grace would say to someone with bulimia, but I can't be kind to myself in the same way."
Recalling how she was eventually prescribed anti-depressants, the 67-year-old admitted that she continued to suffer "further bouts" of depression throughout her life.
"When Brookside took off I was on TV all the time and I hate seeing myself on screen. I became acutely aware of my appearance. So I began making myself sick. I would go to the toilet, stick my fingers down my throat and throw up," she wrote. "It became something I did after every meal. I was triumphant, feeling all my dieting prayers were answered.
"I didn't think what it was doing to my stomach, my gums and my teeth, which began to decay from all of the stomach acid. And that's not to mention what it was doing to my mind.
"I was getting desperate and knew that I needed help because I simply couldn't stop on my own. Anorexia was talked about at the time, but not bulimia. So I didn't think of it as an illness, it was just what I did. It was all so tied up with emotion and the depths of my subconscious."
She added: "I was utterly horrified with what I was doing to myself but I couldn't stop. I would think to myself, 'I'm in my early 40s, not some silly teenager!'"
Johnston described her "journey back to health" as a slow process, as she was again prescribed anti-depressants later.
"Many women have an unhealthy relationship with food - and I think it's getting worse as girls are exposed to unrealistic body images from a ridiculously early age," she said. "But I came through it. And now I am on top of my relationship with food and try to eat healthily and exercise.
"I just hope that by talking about these experiences I can make other women who are struggling the way that I once did know that they can come through it, too."
Sue Johnston's Things I Couldn't Tell My Mother will be released on September 1.
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Watch a clip of Sue Johnston in The Royle Family below: