Have you tried the paradoxical approach to this?
Set aside a period of time every day, such as 30 or 60 minutes.
Allow yourself to indulge your obsession for this time and this time only.
If you find your thoughts drifting back to the object of your obsession outside of the prescribed time, remind yourself that now is not the time to be thinking of this, you have your "obsession time" scheduled at another time during the day.
During the time you have set aside, you can write, fantasize, etc, but you must only do so during the time you have scheduled.
It may take a few tries to get it dialed in, but I have witnessed good success with this strategy. It's right out of the Jay Haley playbook on ordeal therapy.