What to do if your boyfriend cheated on you with your friend?

So recently I've moved to a different high school but it was the same district as my old one. I knew a few people there but not any close friends. Me and my boyfriend decided it'd be better off as friends (my decision) just because things weren't going good but he took me off of all social media. At the school I'm at now, my semi-friend keeps asking me questions about my ex because they were going to talk but not anymore because she didn't know we dated for 6 months. But she said they went on a movie date about 2 months ago saying he met her mom. When I asked her which movie she said "Passengers," which is the movie my boyfriend during the time when we were dating said he was watching that movie with his mom...I'm hurt that he went on a date while we were together and even though it's not the girls fault, she'll still text him from time to time which makes me not want to talk to her but I have no other friends at that school since I've been there for a week. I seriously feel stuck and I texted my bestfriend "She annoys me," and accidentally sent it to the girl and she was like "Omg if u don't wanna be friends anymore then fine," but I made an excuse saying it wasn't for her and she believed it. But I feel like I should get revenge on her because she still talks to my ex and the girl is bi sexual and she said she's had a crush on me before so maybe I can flirt with her and break her heart or just talk to the other guys she talks to but secretly...I don't know but all I know is I'm not texting my ex because I don't want him to think I have to talk to him because we haven't talked in weeks and I want him to end up missing me since I'm his "first" love and I met all his family...Boys suck