Sometimes people become addicted to another person like a drug. You know it's no good for you, it's killing you, but you still crave it. Leaving him may feel like withdraw and that's a horrible feeling. It only gets better a day at a time. Maybe if you think of it like that it makes more sense why you can't let go. You can do it but it takes time. Thinking of him as a toxic, poisonous drug may make him less appealing.