You are sad, so is he. The nature of man (boys) is so mysterious. We can not spend whole of our lives seeing only one face, man wants to go through other faces/people to find the inner satisfaction. Being candid! It is just a sick belief that lovers for years (esp. a boy!) is not supposed to turn towards any other woman in their life. Man gets tired so quickly, avoids what he already has and seeks a new face that could provide him with greater exuberance/merriment/joy. But it is a cycle of failure. Women are recommended to find sometime to avoid what man does (though it is hard for sure), your guy will be back to you after the failure! He actually fails to find the inner satisfaction. It is because "you" were the answer of his question. No other man (man as human) can be the answer of his question. You are made for his inner satisfaction. He will come to you back. Don't reject him for this time, you will regret! but take some time to accept him.