Give the positive guy some more weapons; use humor and common sense to make fun of the negative guy or to cheer the positive one. Don't try to believe things out of "thin air," but talk about common, little things you like about yourself and your life. Just talk to yourself, comment, like you would a friend; about things you like and dislike and struggles you're having, etc. Eventually you'll react to yourself as a friend would and no negative guy can get between that!