Do I stay faithful? Or should I move on?

I have been in love with my best friend for years now. We've known each other since we were kids. We had a thing for each other but we never made a move and her feelings for me slowly faded away. I didn't want to ruin our friendship but I sort of messed it up by confessing to her how I felt about her after her feelings for me had faded away and that resulted in an argument. We still remain as friends, but it hurts to know that she never felt the same way anymore. I haven't done much soul searching before but maybe that's because she has been the one for me but I never realized it until it was too late. I still care about her and she will most likely still need me to be there for her. Should I move on with my life or should I stay faithful to my love for her? I don't mind being single for the rest of my life just so that I can be there for her. I don't know whether she has another guy in mind although I wouldn't be surprised if she does but I know a lot of other guys like her. She is quite the girl after all.