As the world remembered Heath Ledger in celebration of his birthday on April 4, Ledger's family, daughter, and ex Michelle Williams are likely still feeling the pain of his loss.

Ledger's untimely passing in January 2008 at the age of 28 has long been the subject of rumors and speculation.

The star passed away from an accidental overdose of prescription drugs he was taking to manage stress and insomnia.

But many have wondered whether the troubled actor was abusing substances after a difficult breakup with Williams.

Why did Heath Ledger and Michelle Williams break up?

Williams and Ledger reportedly broke up because of his drug use. 

“He had a lifestyle that really wasn’t — at least in Michelle William’s eyes — compatible with raising a child and continuing in that relationship,” People Magazine claimed at the time.

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“Even though their relationship did not work out, she did love Heath — she loved him very much.”

The couple announced in September 2007 that they had split after three years of dating and two years after welcoming their daughter, Matilda. 

Some sources deny that Heath Ledger was abusing drugs.

Friends and colleagues close to the late actor, however, insist that his death really was unintentional. And in the cover story of July 2009's Vanity Fair, entitled "The Last of Heath," they share their thoughts on the end of both his life and his relationship with Williams.

One thing they make very clear from the get-go is that Ledger was not, in their opinion, an addict.

Cinematographer Nicola Pecorini, who worked with Ledger on his last film, "The Imaginarium of Doctor Parnassus," revealed that Ledger used to smoke marijuana regularly, but "went clean as a whistle" when he thought it was becoming an issue.

And vocal coach Gerry Grennell, who worked and lived with Ledger during the shooting of "The Dark Knight," insists that Ledger didn't even drink. 

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"Everyone has a different opinion of how he passed away," Grennell says. "From my perspective, and knowing him as well as I did, it was a combination of exhaustion, sleeping medication [prescribed for his chronic insomnia]...and perhaps the after-effects of the flu. I guess his body just stopped breathing." 

Heath Ledger and Michelle Williams reportedly had different perspectives on fame.

As for his relationship with Michelle Williams, Terry Gilliam, who directed Ledger in "Doctor Parnassus," says that it began to unravel during the Oscar campaign for "Brokeback Mountain."

"That was the moment when it changed when he realized 'Uh-oh, we perceive the world differently.' He didn't care about things like those awards."

Ledger did, nonetheless, care about Williams and their daughter, Matilda.

According to Pecorini, "Heath was always blaming himself [for the split], asking 'What did I do wrong?'" Gillian seconds that sentiment, saying that initially during the breakup: "He really thought he could do the right thing. He was trying to be decent and graceful, give her whatever she wanted..."

But apparently, the stress of custody proceedings and lawyers eventually took their toll.

And according to Gilliam, Ledger fell apart. "He said, 'Just f--- all of you. I'm not giving Michelle anything.'"

Sadly, his words turned out to be prophetic.

When he died, Ledger left behind a will that pre-dated his relationship with Williams and thus, left nothing to her or Matilda.

However, his family has made sure that the two have received everything they believe Ledger wanted them to have by gifting his fortune to them.

Busy Philipps believes Heath Ledger and Michelle Williams would have gotten back together.

Philipps, who is close friends with Williams, wrote in her 2018 memoir "This Will Only Hurt A Little," that their 2007 breakup "wasn't the end of the story for them."

“They were so young and that baby was the light of both of their lives. They were just working all the time and it was complicated,” Philipps wrote.

“Sh*t is always complicated. Especially when you’re twenty-eight. And movie stars. With a baby.”

Michelle Williams has spoken about her grief after losing Heath Ledger.

“I am the mother of the most tender-hearted, high-spirited, beautiful little girl who is the spitting image of her father,” Williams said in a statement following Ledger’s death.

“All that I can cling to is his presence inside her that reveals itself every day. His family and I watch Matilda as she whispers to trees, hugs animals, and takes steps two at a time, and we know that he is with us still. She will be brought up in the best memories of him.”

In a 2010 interview, she said that she is primarily focused on raising the couple's daughter and admitted that she has struggled to move on after his passing.

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Kristin Meinzer is a news and entertainment writer for Newsweek, a podcast host, and author of two books.