Simply go to somewhere quiet, take a deep breadth and tell her
straight/ By being a good guy, DO NOT waste her time with you no
longer loving her.
That is why some guys are bastards, don't love the gal, still stick
with her, giving her false hopes, then few years down the road,
then tell her. You are wasting her youth and there might be some
better guys out there better then you and if you really love her,
you don't want to do that to her right?
To lessen the pain and hurt, tell her as early as possible, imagine
you drag this on for another few months, it only makes it harder
for you to break up with her and also the extra months of love she
puts into you only makes her pain even more.
Good luck to you 
Hope this helps
