How do i break the news to my girlfriend of 5 years?

My girlfriend and I have had a great relationship over the past 5 years. She started out as just a friend of mines back in 11th grade. But after a few months into our friendship i caught feelings for her and she for me, so we started going out. She really gets me and i really loved this girl, she was like my long lost other half, most people dont meet this person in high school nor do they keep them through 4 years of college but we stuck it out. She has even been giving me hints that she wants to get married. And this is where everything falls apart, dont get me wrong i still love her but im not In love with her anymore. Its been like this for the last 4-5 months. She hasnt done anything to make me feel this way its just that i dont have the same feelings for her that i used to. The reason why i waited so long is because i thought that those feelings would eventually fade and i would fall back in love with her again. But its been almost 5 months and im still not feeling it. The last thing i would want to do is her hurt because she is still my best friend, but i know that telling her this would absolutely crush her. How do i break up with her as painlessly as possible? I still love this girl just not the way i once did. I want us to still be able to be friends and have our old friendship back after this.