HI, I know this reply is years late, but I just found this forum. I always had to have an obsession with someone, since the age of 12. First it was the boy who lived across the street, then my teacher, ex-boyfriend, my doctor, etc. One obsession would fade, another would begin. I HAD to have this in my life. I finally realized the purpose was to keep my mind occupied--give it something to focus on to keep me from getting very depressed.
It really has nothing to do with reality or the person you're obsessed with. You can train your mind to focus on goals and projects instead (which might actually get you somewhere, unlike obsessing on a person). It takes practice, but can be done.
Also, realize that your imagination has elevated this person to an unrealistic level, like a god. See them for the flawed, average being they really are.