I hooked up with her best friend!

I had a casual relationship with my ex-girlfriend’s best friend and don’t know how to tell her this. It wouldn’t have been a problem if she was still out of my life, but she has been messaging me and talking about getting back. If she finds out about this, she will never come back to me and I really want to try and make this work. It’s obvious that her friend has not said a word to her, so I don’t know if keeping quiet about it is the best approach. I will constantly be on edge about her finding out through someone else, which is why I am nervous and don’t know what to do. If I tell her what happened, I am taking the risk of losing her forever. Please help.
Your ex’s best friend may have her own reasons for not saying anything, but you can’t hope to have a healthy relationship by keeping something like this hidden. If you are constantly afraid, it is a sign of guilt because you believe you did something wrong. You were not in a relationship with your ex at the time, and she needs to understand that. If she chooses to not forgive you, that is her prerogative, and she is allowed to feel the way she does. Trying to give this relationship a second chance only makes sense if you approach it with honesty because withholding something that may destroy it at a future date only deflects the risk you are worried about taking now. If you don’t want to lose her forever, you are only postponing a difficult conversation. Sweeping things under a rug doesn’t resolve them.

My friend and I like the same girl. How do we go about this without letting it affect our friendship?
Why assume she may be interested in either of you? If you want to save your friendship, start treating this girl as an individual with a mind of her own, and accept that adults are allowed to make choices.