Irresistible (2006)

Sophie Hartley is convinced that she is being stalked. She becomes increasingly certain that her husband's beautiful co-worker, Mara, wants her children, her husband and her life. But no one believes Sophie. Forced to prove her sanity, Sophie grows increasingly paranoid - but is she imagining things? Sophie becomes completely caught up in her obsession, turning stalker herself - and makes a discovery more frightening than her worst fear.