What am i supposed to do? Please help..?

Early this year, when i just broke up with my girlfriend, i met this girl online whose relationship was on the rock and not soon after, they broke up. We chatted, exchanged phone numbers and clicked well instantly. Whenever she needed someone to talk to, i was all ears. Even so, we never met in real life. Somewhere in June, she confessed that she started to develop feelings for me. I suspected that she was still in love with her ex. Being so gullible of me, i bought her words and tried to give her a chance. Deep inside, i do like her too. In the end, we decided to meet. The date was ok and we still keeping in touch after that. I did notice changes in her behaviour. She did not sound affectionate and caring as she used to be and not even once she ever mentioned about us being together.Things started to fall apart when she started to go out with her ex again. It took days for her reply my text messages with excuses such as she was busy with her work and a lot of stuff. This went on for few weeks. She would only come to me when she had problems at work or with her ex. Finally, she confessed that they are now back together, and then even have planned a vacation end of this month. I felt so hurt inside but i kept my cold by still retaining the friendship. Now, i already give up and try to minimise our contact. After a while she has started to realise that we are no longer close as before, that i have become so cold and distant towards her. She told me that really misses the old me, misses my text messages, misses my care and she could not afford to lose the friendship. I could see that she is trying hard so that we can be close as before. She keeps on asking my sudden change and cold treatment. What should i do? I don't think that things will be the same again. I feel used and cheated. How can i ever regard her as a friend when i still have feelings for her and i don't want to be the one who ruins their relationship? How to put an end into this? I do not have the gut to tell her the truth as i do not want to hurt her feelings and i know that if she is mad over something, she would do something crazy.