I’m in your same situation.
My ex and I broke up. Not too long ago after that I find out she’s talking to someone else and let me tell you, dude isn’t the nicest looking.She downgraded with me too big time! As far as I know, it’s been a month and she still has contact wth him. I also believe he’s a rebound. The last time she saw me she told me that maybe in a month or so we can try it again (safety net).
I felt just like you when I found out. So confident and happy that she downgraded. But best thing to do is forget about the other guy becuase even though you don’t see him as a threat your mind will still wonder what they do. Believe me it’s unnecessary pain. So just focus on yourself only. As far as you know there is no other guy. NC will help you become more desirable
Good luck