The two stars toured the world together on Miley's Bangerz tour and Lily admits she put huge pressure on herself to shed the pounds because she was sharing the stage with "this girl who was younger and more attractive than me."
She quickly became hooked on drugs, hit rock bottom, and pledged to get her life back on track.
Now healthy and sober, the "Smile" singer tells "The Recovery" podcast she turned to drink, drugs and prostitutes as her life spiralled out of control.
"About six months after my youngest was born we ran out of money and I had to go back out on the road again, but I was 14 stone and did not feel like a pop star at all."
"I started taking a drug which is like speed to lose the weight and I got addicted to this drug because it made me feel invincible. And then I ended up on tour in America supporting Miley Cyrus... It was when she was doing Wrecking Ball - it was a highly sexualised tour. I had just spent the last three years pushing babies out. It couldn't have been less what I felt like (and) I had never supported anyone..."
"I was re-entering this phase of being a pop star again but not doing it on my terms any more. I was supporting this girl who was much younger and more attractive than I felt. I just started acting out in all manner of ways..."
Allen reveals she turned to drink and drugs, started cheating on her then-husband, Sam Cooper, and even contemplated taking heroin, "I was thinking, 'I think I have got a drinking problem. And none of this acting out is working any more. Maybe I should try heroin... I had been in a scene, where I had seen what happens to people who use heroin (and) when that thought popped into my head it was time to, you know, confront my demons. That was about five years ago. And I started recovery."
She signed up for a 90-day recovery programme, determined to get clean and back on track.
"I did 90 meetings in 90 days and I got clean and at that time I wasn't really committed to the programme as a lifelong thing. I just wanted to get to six months so at least I know I can stop this when I need to," Lily adds.
"At six months I started drinking again and almost instantly I lost everything. I lost my marriage, I lost my house that I had worked 10 years to buy. My career started sinking. And I lost all my friends... I didn't have any of my friendships any more. I was so resentful and so angry all the time, I really felt that the world owed me stuff and I got the raw end of the deal. And that went on for another four years and then I ended up back in The Rooms (recovery centre) again."
She's now a regular at meetings and journals her thoughts and feelings so she can stay on top of her urges and stay sober - for her kids.
"I felt so guilty about neglecting my kids in those early years of their lives and having to go off on tour and misbehaving in the way I was," she explains. "I really have a great relationship with my kids now. I am there to pick them up at the school gates whenever I can be and I am up dropping them off in the morning and I make them dinner."
"They come to me when they've got problems. That's golden to me. They are confident little girls and I don't feel like... they are going to turn into drug addicts like I did. They seem like they are on a good path."
"He is sober, he has been sober for 20 years now - we are thinking of what we are going to do with the rest of our lives... I feel a sense of... I felt like I really had to prove people wrong in the past and that people had all these preconceptions of who I was, whereas now I get to wake up in the morning and interact with my children and make them feel loved and protected... and then I can go off and do my work. It doesn't have to be a number one single. It's not a failure if it isn't a number one single. I am just lucky that I am able to be able to do this and I get sent the odd designer handbag. Life is good."