I need help not to be a jackass

OK so I went on this school trip with two off my friends and along the way I end up making out with a girl. I am a guy so i am straight but I hate these fucking lads who go around treating girls like shit but i may have turned into one as I really didn't feel anything with her and I have been avoiding her in a way. So the trip ends and I end up being to much off a wimp to do any thing about us two. Now she thinks I am into her and a voice in my mind is telling me to run and I am doing it. So could anyone give advice on how to let her down easily and hurt her in the smallest way because she is a nice girl and i don't want to hurt her. Please give some advice I need it.