I feel very obsessive lately about this older man and I get depressed when I can't see him. I'm going to tell my doctor. How should I stop obsessing?

Obsessed. Not sure what your age is and how this man is related to you, but I am sure your doctor can help you sort it out.
How about a book? There may be other issues regarding why you may be obsessing, like issues related to a father figure based on your childhood. That may take some time to work thru with a therapist. Meanwhile, check out the book: stop obsessing by edna foa and good luck.
Need more info. Not sure what you mean by obsessive. Do you mean possessive? Do you think about him to such an extent that it interferes with your life? Do you have a relationship with him? How much older is older? Is he married? Do you get jealous if he talks to others? When you say doctor, do you mean a therapist or a primary care doc? Talking with a therapist could be helpful. Good luck!