<human>: Give me the gist of the text in a nutshell

Long story short, I've fallen for my best friend. She's actually the greatest girl I've ever met but we've always lived far apart and she likes another guy who she's dating. A lot.  We're all 20 years old but she wants to get married by 25 and he agrees. She's happy.

I've obviously never told her how I felt and was honestly planning on telling her when I next saw her in person. But she just told me that she wants to marry him. I can't tell her how I feel and risk ruining her happiness (her life's been pretty complicated and its finally pretty simple). That and I know that I have absolutely no chance with her (proverbial "friendzone").  So I want to stop being friends.  I know that sounds horrible but the more I think about it I come to the realization that this friendship is bad for me.  It's not healthy for me.  Not a day goes by where I don't think about her or talk to her. I need this to end.

My problem is I don't know how. She's actually my closest friend. The only person I've ever felt 100% comfortable with. She's the only one on this planet who knows me for who I am. And I don't know how to stop being friends with her.
<bot>: Fell for my best friend who I have no shot with and no longer want to be friends with because it's unhealthy for me.