The 28-year-old mum of daughters Ethel Mary and Marnie Rose said she is determined to shed the pounds after feeling particularly self-conscious at a friend's wedding recently.
A friend told Closer magazine: "With her comeback getting nearer, Lily is becoming frustrated. She really wants to lose weight all over, but especially on what she calls her 'mummy tummy', thighs, back and face.
"She'd love to be a toned size 10 when she hits the limelight again, but she can't seem to get enough motivation to eat healthily.
"She'll starve herself for hours, then hit the sugar because she'll be so hungry. There will be weeks where she will work out every second day, but then she won't go to the gym for two weeks."
The magazine says the 5ft 2in star, who is married to Sam Cooper, is feeling very self-conscious.
At a wedding last week, where her one-year-old was a bridesmaid, friends said: "She spent a lot of the day tugging at her dress and adjusting it. She says she felt frumpy and has been saying she needs to buy some new clothes but refuses to until she's a size ten again."
More on Parentdish: Let's stop trying to emulate celeb mums. So what if we have some extra curves?