John Travolta is "devastated" after losing his wife Kelly Preston, who passed away on Sunday after a two-year battle with breast cancer.A source told Us Weekly that Kelly Preston's family is finding it difficult to deal with her demise as the 57-year-old was the "rock, the heart, and soul who kept the family united."The source also said that Preston and John Travolta "were true partners," and the "Grease" star has been left "devastated" after losing his wife of 19 years.