How can I be nice to my ex's girlfriend?

The girlfriend is moving out of state this week, to the same town my ex moved to (several states away) He first slept with her 10 years ago, and denied a relationship with her despite evidence to the contrary. Now he mentioned to my son he worries about introducing his "new girlfriend" to me at a graduation party. We all know this is not a new girl at all, and I am still hurt he lied to me all these years, and is still lying about. How can I get over my hurt and be civil and mature about this; how long does it take to put the past behind and not dwell on the pain of betrayal? I am glad he is gone, but the lies still make me angry