How do I tell her???

Laticia

So my best friend recently broke up with her (ex)bf of 1 month for his best friend(new bf) and kinda crushed him. Then he started talking to me and we met up at this party and ended up fucking in his car that night. My best friend found out about it and got mad for like 2 mins and then was fine with it but a few days later she decided she didn’t want us together and told me to not go for him. She said it’s weird because we all hang out in a group together but it’s already weird because she’s with his old best friend. She told me she doesn’t have feelings for him and that she just doesn’t like us together and I feel like that’s really controlling. Anyways I’m dating her ex now and I don’t know how to tell her?? I don’t want to loose her again we just got back to normal:/