Hi Claudiii,
Working with your ex would test anyone’s enlightened nature LOL. Even if you were completely over him, the niggling memories would be there. This is why as a general rule people don’t date their work colleagues. Short of going to another branch, working from home for the company or switching jobs, there’s not a whole lot you can do.
Maybe don’t talk to him unless you have to, be polite, tell him, “No, I don’t want to hear about your new girlfriend/amazing weekend” (God forbid he says something like that).
Just think of him as someone from your distant past who’s a different person now (he eventually will be). Or that this is his identical twin Skippy.
Hang in There,
Inky