Just sit her down at home and tell her you are unhappy in the relationship and want out. Let's face it, break ups have never been easy and someone is bound to get hurt. If you are truly afraid of hurting her, than you will just have to settle for what you have in the relationship and make the most of it. I don't see the point in wasting anymore time. By the way, the 20's are not always the best years to find the perfect match. Most people don't find the right one until the second and third marriages when they are well into their 40's.
She will find another guy. It won't be the end of the world.