"Heath's drug use was something that affected them profoundly," says a source

Until their split last September, Heath Ledger and Michelle Williams were a funky young couple living in a renovated Brooklyn row house who adored their little girl, Matilda. Now, “You can’t even imagine what Michelle is going through,” a pal of the actress’s tells PEOPLE for its new cover story.

“She is so heartbroken,” says the friend. “It’s one thing to lose someone so close to her, but the fact that she lost Matilda’s father is crushing. Her family is getting her through this.”

Many who spent time with Ledger in the last months of his life paint a portrait of man whose demons drove him away from the family he loved and into a lonely, nomadic life of insomnia, casual affairs and, according to several sources, serious drug use.

“Heath’s drug use was something that affected them profoundly,” says a source close to Williams. “Michelle didn’t want their daughter around that. She was very clear that Heath loved his daughter and was a good father.” But “she couldn’t always rely on him.”

For the complete story of how Heath’s partying affected his relationship with Michelle – but never changed her love for him – pick up this week’s issue of PEOPLE, on newsstands Friday