I had found myself in exactly the same position a few months ago..
this girl i was seeing for a few weeks had already planned out this huge wedding, house, car..dog etc.. she was a few years older than me - granted.. but she was nuts.
Gorgeous... but totally loopy..
It also helped that lived quite far away

like yourself zoomzoom..
call her up - and just tell her bluntly "i've found someone else.. i've known her for a long time - im sorry"
but you have to make it clear the girl on the recieving end - that your time with her was special you will never forget it.. just say it, that way; you don't come across as complete scum. (i mean that in the best possible way!)
if you decide to go with this, don't expect any more sex from them..
Good Sargin'!
Illusionist
x x x