obsession
An almost insane desire, longing, or burning lust for someone or something. You feel like without that thing or person, you are not complete and can't cope, or- in extreme circumstances- live, unless you have the object you desire.
Example:
Getting back with my ex-boyfriend has become such an obsession for me that I lose sleep trying to think of ways to lure him back into my arms.
Getting back with my ex-boyfriend has become such an obsession for me that I lose sleep trying to think of ways to lure him back into my arms.