gams,
My sense of your issue is that this is territory for psychotherapy not an online forum with us amateurs *grin*

What comes to mind for me to offer is for you to get a pen and paper and write down all the qualities of that person you are obsessing about. What about him/her that appeals to you so much? Go beyond the superficial qualities of looks and appearance though those are important to note as well. What first catches your attention when you first met him/her? What drew you into wanting to know that person more?

After devoting time to that, see if those qualities remind you someone from your past, perhaps from your family-of-origin?

If nothing else, this exercise will help you become more mindful of what you value in a person (and hence in yourself).

Mark