I'm confused about what I should do about my relationship.?

I'm 15 and my dad is in the Army so my family is constantly moving. I previously moved to Arizona for 6 months and started dating a girl in the neighborhood. As time went on I started to actually care about her. I started walking her home from her school (I was home schooled). Then one night she gave me a kiss on the cheek and a couple nights later we had our first kiss. As time came closer for me to move I had fallen in love with her. I did so much for her and did my best to make her happy. One time I spent a week washing cars so i could buy her a pair of custom converses that had ILYM on the back. It stands for a song "I love you more" that I played and she was so touched by it we hugged for about 10 minutes. I don't think she has ever felt like someone has cared for her as much as I have not even by her parents. So as time got even closer I had no doubt in my heart that I loved her. so I told her I love her and she said she didn't want to her me say that again. I soon found out that she didn't want me to say that because she knew I was moving again soon and that it made her cry whenever she thought of me not being there. She told me she had spent the whole night I told her i love her crying because of how it made her feel. She said she wouldn't do long distance because she had seen long distance relationships crumble with cheating and other stuff so I made sure she didn't want to change her mind and she said she was sure. So after a while the time had come and we had said our final goodbyes and she took me over to a corner and told me she loved me too and I gave her a hug we kissed and broke up. My family and I moved to Germany. we were able to keep in touch with Facebook. I sent her a message one day with a link to the song "I love you more". The next night I was talking to her and she just started exploding with feelings. She said she really loved me and that it killed her for me to leave. She even told me that she had REALLY thought about it and told me she had thought about getting married someday. I told her that was exactly how I had felt too and that it drove me insane keeping it inside. but then the unthinkable had happened!! she had started to date another guy! and when that didn't work out she started dating yet another guy! Her friend told me that she was doing this to make her feel better, and so she could have other experiences with other people. This really just killed me inside. and that is where the situation stands as of now. I would really appreciate any advice on what to do, I'm really just confused and don't know what to do.