How do you "break up" with friends?

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So I have a friend that I have been close with since about first grade. We have been together since gradeschool, continued to highschool together, and are now at the same college. We're actually roommates now in college. For the past year I have felt like we have WAY less in common than I thought we did. I have done alot of growing, changing, and maturing as a person.... and she is still stuck in high school I guess you could say. It has gotten to the point where I don't even feel like I value our friendship anymore. My question to all of you guys is if you've ever been in this situation, what is the best way to get out of a sinking friendship?