made a mistake. what do I do

My boyfriend and I got back together last week after being broken up. Right before we got back together, I started talking  to a guy on a dating app. When my bf and I got back together, I continued talking to this guy from the dating app and now me and this guy are supposed to go on a first date on Friday. Idk why I agreed to the date. It's so wrong and I don't want to go on that date anymore. How do I cancel with him, without being a jerk? Please help. I don't want to do anything that would hurt my relationship with my current boyfriend, so I need to cut ties with that guy.