It's been over four years since Michelle Williams' ex-fiance Heath Ledger died of an accidental drug overdose -- and, after several stalled relationships, Oscar-nominated actress and single mom Williams, 31, is ready to get serious with pal-turned-beau Jason Segel.

"Michelle told one friend, 'I love him!'," a source says in the new issue of Us Weekly, out now.  But pals say it wasn't an easy journey for the My Week With Marilyn star, who "had a lot to recover from" as she mourned Ledger and cared for their daughter Matilda, now 6.

The sudden death at age 28 of the troubled Brokeback Mountain star "took a huge toll on her," another insider says. "She is a romantic and and always felt partly responsible for not being able to help Heath."

But 5 Year Engagement star Segel, 32, promises a fresh start. "Jason has already shown her his commitment to her," a Williams source says.

"She doesn't want to repeat the same mistakes she made with Heath," a Ledger source adds.

For much more on Williams' serious new romance -- Segel's reputation as a "playboy," his bond with Matilda, how she honors Ledger's memory -- pick up the new Us Weekly, on stands Friday.