>1 she lives on the other side of the country and has a boyfriend.

This are two major reasons why you should stop thinking about her. The -how- is to talk to her only if she comes talking to you. Do not approach her. If she comes talking to you, well, you won't ignore her - but try to keep the conversation as neutral as possible, do not show enthusiasm - with time, your "obsession" will get tamed, and eventually you'll stop thinking about her, just...slowly back away from her.
The only cure is time.