updated 01/31/2012 AT 5:10 PM ET
•originally published 02/01/2012 AT 8:00 AM ET
Never shy about flaunting her bikini body onscreen and off, Demi Moore had long been regarded as Hollywood’s most age-defying beauty.
But as the actress approaches her 50th birthday – and her marriage to Ashton Kutcher, 33, began to crumble – several sources tell PEOPLE in this week’s cover story that the 49-year-old star battled deep feelings of insecurity. In fact, sources say the star was barely eating and partying wildly before her drug-fueled breakdown.
“As Demi got older, she convinced herself that she needed to stay young and skinny to remain attractive to her husband,” says a source who knows the former couple.
Continues the source: “She needed reassurance all the time that she was hot and sexy.”
Another source says that Moore, who is now seeking treatment following her Jan. 23 hospitalization, “was always fixated on being young. And the only thing that tied her to that, as she got older, was her marriage to a younger guy and hanging around young people.”
For much more on this story, including Moore’s bizarre behavior and her ex-husband Bruce Willis’s attempt to help her, pick up this week’s issue of PEOPLE, on newsstands Friday