Obsessed

What do you call it when you fiend for something so that it's all that runs through your mind twenty-four seven. When the thought of someone else having it in their possession besides you causes you to go insane. So much that you want to lock it away and hide the key because no one would ever be able to get to them but only you? What do you do when that something is a person? Some may call it to love but to a certain extent, it's an Obsession.