It sounds like she isn't actually everything you've ever wanted in a woman, and that deep down you know that something integral to a relationship is missing for you.
It always sucks when you're in a relationship with someone who is actually a relatively great person, but there's just enough little things that add up to you being unhappy with the situation. Obviously it sucks to have to hurt someone by moving on, but maybe you should consider that you're doing them a favor by not dragging them through the gutter with your disappointments in the relationship.
There's nothing wrong with amicably ending a relationship. The hard part is convincing the other person that while they're a good person, they're just not a good match for you. They don't understand it, and you can't really explain it because there's some missing integral piece of communication between you two. You say what you mean, but she doesn't understand it.
Anyway, look. If you're not happy in the relationship you should respect her enough to end the relationship quickly and allow her to move on with her life. Otherwise you're wasting her time and hurting her more in the long run.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."