I think i don't love my girlfriend anymore. How do I end it?

My girlfriend is amazing, really nice, intelligent and overall a great person. But I just don't feel like I love her anymore. I've had my doubts for a while now and when she sent a message today, saying how much she loves me I realised that I just don't feel the same. I wansn't happy to receive that message, I just felt guilty for not feeling the same way for her anymore. I feel it would be unfair to string her along and looking through similar posts on here strengthened my decision to break up. I just don't know how. I feel miserable just thinking about hurting her, especially knowing she was hurt before. What makes it even more difficult is that we are in a long distance relationship. Do I end it in a phone call? That just doesn't feel right to me. But how would I do it if I did it on a visit? I mean I can't break up with her and then stay there for a few days but I also can't stay there for a few days as if nothing was wrong and then break up before I leave. I'm visiting her next weekend for her graduation night and I don't want to ruin that for her. What do I do?