I Don't Want Us To Be Friends Anymore? Please Help Me?!?

I have this 'best' friend. Or so she thinks. We've made tons of memories together and have gotten together on weekends and had sleepovers and all that jazz. I do enjoy spending time with her; only when she's in a good mood. Lately, I've just been getting sick of talking on the phone and having nothing to say and making an effort to keep in touch, it doesn't come naturally. Like, I have to force myself to talk to her or make plans. But now she's all "I love you and we have to stay best friends because you bring so much joy into my life <3" And yes, no doubt I love her to death but I think we need to part ways.. How do I tell her this without being mean? Because, I'll feel really bad if I just stop being friends with her, although that's what I think is best for both of us.. Help!