Most Helpful Girl
Just break up with her. No need to prolong it. It'll hurt her, but staying with her would be unfair to the both of you. Seeing as it seems like she's still very much into you, I think you should do it slowly. Let her get used to not having you around. Act cold, but don't be hurtful. Basically, just prepare her for it gently. Then break up with her, but again, try not to be hurtful. Just say that you fell out of love and be honest about what you feel. Good luck!
Most Helpful Guy
The best way is always the truth. You are going to hurt her and that can't be avoided. If you wish, you could stay friends if she is interested. be honest, but not blunt.