Stuck between a rock and a hard place! What do I do?!?

Ok, I've been in a relationship for 13yrs. Recently she was away for a year overseas, while away we both changed some. While she has fallen even more in love I have found myself falling out. I ended up falling for someone else who also fell for me. I remained faithful but all I can think about is this other woman and how much I want to be with her. My current girlfriend is my best friend, I love her but I'm not in love. I don't want to hurt her or lose her completely but I also don't want to stay if my heart is not in it anymore. What do I do? Who do I chose? How do I let her down easy without ripping her heart out? I'm all she has but also don't want to stay out of guilt. I'm so lost right now!