mate my advice is, you cant change a womans mind when she has it set on something, your only 18 mate and i know its hard we've all been through break ups good and bad and niether are easy, if there is still a glimpse of hope then try and convince her your the right person for her, but if she has valid reasons why she wants to break up there really isnt anything you can do. like many others will tell u there is plenty more fish in the sea, ur 18 still plenty of years ahead of u.
why were u thinking of breaking up with her? why had u lost interest? do u really think with ur gut feeling that there is more to get out of this relationship? or would it be better to count your chickens and have the good memories of the last 3 years? u dont want to end on bad terms believe me, when you end on bad terms things start running into your head, what she doing now? where is she now? blah blah blah, and believe me when you get into that state of thinking there is no easy way out of it. can you still be friends with her? you never know in the long run both of ya's might realise "hold on maybe he/she is the right person for me?" u've been going out with the same girl since you were 15, maybe a break would do you good, play the field, meet new people maybe people you would never meet if you were still with her, u'll be surprised how many interesting people you meet after a break up... but keep your head up mate remember its not the end of the world, you may be hurting but everyone goes through it, just remember your not alone talk to your family, friends, just loved ones they are the best people to help you through these types of things, i know i owe my life to my whole family (friends included) god knows how i would have coped, but stay strong dude.