He won't let Go

Hi, I am new to the site and find that there are lots of folk in the same position, feeling the same range of emotions - anger hurt revenge etc etc but would like to know if anyone can suggest how to handle my ex in so much as he wont let go. He  had the affair & left ( I am not apportioning blame) but he continually 'gets inside my head'  as though he thinks in some way I should still be his wife i.e. still sympathsise with how bad things are for him.  I can cope usually by simply avoiding him but we have to share info about the kids. (are independent- late teens) I need to move on and try