Gwyneth Paltrow feels a “deep sense of impending grief” about her children heading to college. The 51-year-old Goop founder has daughter Apple, 20, and son Moses, 18, with her 47-year-old with ex-husband Chris Martin, and is stepmother to producer spouse Brad Falchuk’s children Brody and Isabella. The lifestyle guru admits she has no idea what life will look like once her children fly the nest because motherhood has been her whole identity and “guiding force” for the past two decades. Moses and Brody are heading to university later this year. She told The Australian Women’s Weekly magazine: “I…