I need help with boy and friend drama

A friend of mine recently asked me out. (15)He’s one of my best friends and we are in a group of very close friends. I have never been good at letting people down and especially since he is such a good friend, I said yes out of panic. I have had feelings for another person for a long time and I feel like I’m missing a chance with them right now. But, I’m his first girlfriend and my friends keep telling me how much he likes me but I don’t feel the same. I’m going into freshman year in less than a week and I don’t know what to do.