Most Helpful Guy
i would say this is because when you are apart you have an image of him in your mind and when you see him you see the real deal.
Most Helpful Girl
Maybe a case of distance makes the heart grow fonder? You also likely find comfort in creating a fictional version of him when away and want to believe in the person you thought he would be and not the person he really is.
The only way to move on is to distance yourself from him completely.