This is to save all women I date in the future the hassle of coming up with their own original, poorly-phrased break up excuse. It's always the same excuse anyway, why not practice it ahead of time, so it flows nice and smooth.
I'm just not getting the emotional fulfillment I need from you, and I feel like you put up walls between us that you refuse to take down. I haven't been happy for a long time, and I know you've danced in circles and bent over backwards trying to make me happy since we discussed this before, but I just don't think I can find true happiness with you. Maybe I need to learn to be happy by myself before I'll be able to find happiness with someone else. You're so wonderful, and I love you, I'm just not in love with you. I don't want to hurt you, and that's why I need to stop hurting both you and myself by ending this relationship.
You may need to modify that line about not being happy for a long time if we've only been dating a short period of time, but any way you want to use it, it's much classier than just disappearing and never speaking to me again although when it comes down to ending the relationship, that method is just as effective.