When to tell my friend I'm moving away?(kind of longwinded, sorry)?

I'm about to enter my last year of highschool, the following autumn I am moving to Brooklyn to work full time, and then enroll in the spring term at college. I don't live ridiculously far from NY as it is - an hour and a half(provided traffic isn't retarded).

I've told a select few people about it. Recently, my parents. This is kind of a 'get out or I'm going to go insane and end up homicidal' type of deal, I personally feel like I have no other option other than going somewhere either very far, or very different. So it is definite. The time may change slightly(we're talking give or take a month), but I am 100% positive I am going.

My issue is a few of my close friends. Particularly one. She is perfectly content to stay, is attending local college, and is eager to rent out a place to live with me and a few other people. Obviously, we're different types of people - I'm pretty confident I'd need heavy medication to keep me in the general area any longer than I need to be. She doesn't want to go anywhere - much less a crowded, expensive, dirty city.

She'd be hurt if I just dropped everything and abandoned her and everything - so I've been avoiding the subject, but I know I'll have to tell her sooner or later.

I have a number of OTHER commitments to take care of(I ride and show horses competitively, and own a horse, who's living arrangements I am trying to sort out without selling him - my parents have spent a lot of money, my coach has put in a lot of effort, and she's going to be very disappointed when I tell her I'm going to have to take a long hiatus), and I know I can't just drop everything and run away - but there are so many people and things that I feel obligated to that it's intimidating to think of.

I feel selfish for leaving, but at the same time...whenever my friend talks about 'the future' and I associate it with 'this place', I want to rip my eyes out.

SO. suggestions?