Michelle Williams put pressure on herself to find a new serious relationship after Heath Ledger died because she wanted a sibling for their daughter Matilda.

The 'My Week with Marilyn' star always hoped the youngster, now six, would have a brother or sister close to her in age and so felt she needed to move on quickly following the death of her former fiance in January 2008.

The actress - who embarked on a year-long relationship with director Spike Jonze months after Heath passed away - explained: "Because I really wanted, and I really expected or imagined, that Matilda would have siblings that were close to her age. I wanted that for her. But I couldn't make that happen. And now that she's six that isn't even a possibility anymore. So something that was making me feel impatient, that's been removed. For whatever reason, that's not our luck, or our path."

Although Michelle has found life tough since Heath passed away from an accidental DRUGS overdose, she thinks it is harder for her daughter because the youngster will have few memories of her father that she doesn't share with anyone else.

She said: "You know, as hard as certain things have been for me, it's been harder thinking about how things will be for her. I have a lot of things that she doesn't, and some of what I have I can give to her--the memories that I have, the objects that I have, the physical reminders that I have, the stories. But she won't really have any that are solely..."

Michelle and Heath met on the set of 'Brokeback Mountain' in 2004 with the actress giving birth to their daughter in 2005 and the 31-year-old actress believes their relationship developed so quickly because they subconsciously knew they didn't have much time together.

She told America's GQ magazine: "Our initial meeting, the circumstances of how we first met, were cosmic or something. Yeah, a lot of things happened at once. It's a bit like: We had a lot of things to do, because we didn't have a lot of time, or something."