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