LOS ANGELES-Jennifer Anniston reportedly worries about her engagement to Justin Theroux.

The 45-year-old actress and her Wanderlust co-star have been together for nearly three years.

Recently the couple have been hit with distance as Justin films his TV series in New York. It is thought this gap between them, along with the actor temporarily moving into the home he shared with his ex Heidi Bivens, has caused Jen anxiety.

"Jen says she has nagging doubts she and Justin will ever marry, or start a family," a source told British magazine Closer. "She loves him and he visits her whenever he can, but their schedules are hectic and, when he's in LA, they often bicker. Jen also thinks that him moving back into the house he once shared with his ex is a step backwards."

Before getting engaged to Justin, Jennifer was married to Brad Pitt until 2005 and briefly dated Vince Vaughn and John Mayer.

Even after reassurances from her fiancé that he and his former flame's now-relationship is simply platonic, the blonde beauty still reportedly has fears.

"Justin's ex lives near their old house, but he's assured Jen that he and Heidi are just friends. However, Jen is worried that the distance could break them," the source added.

Jennifer has made her love for Justin no secret. Last December she couldn't contain her feelings for the 42-year-old star. "I'm so happy - I feel the most beautiful when I'm just at home with Justin," she smiled.