SIENNA DUMPS FIANCÉ!

Sienna Miller continues to be unlucky in love.

The 38-year-old actress, who’s dated a bunch of Hollywood heartthrobs over the years, has broken off her engagement to her younger fiancé, Lucas Zwirner, 29, the son of an art gallery owner. “To be honest,” says a source, “their relationship turned sour months ago. They hoped the engagement would bring them closer together. It didn’t. Lucas was dragging his feet with the wedding plans, there were jealousy issues, and they were fighting a lot. Sienna finally had enough and called it a day.”