My Ex-Girlfriend Won"t Let Go!

Often when a relationship ends it isn't over for both people.
There is a "connection" that they try to hold onto after the breakup.
And by re-inserting themselves into your life (now that you are single or seeing someone else) they are trying to regain the control they had over you during the relationship.
To help them "let go" you have to decide if their attempts to talk to you, to see you, to be with your friends that you have in common is simply a way to maintain their own life or to get back into yours.
This can be a hard task or an easy one depending on the ways you keep "running into" them.
You should draw the line at their attempts to call you, email and instant message you.
Let those calls and emails go unanswered and unread.
Having common friends should not force you to choose which friends go with who after the breakup but you may want to stay a little closer to home to give the other person more time to heal and less time spent near you, in the company of others.
The healing from a breakup can only happen when there is a clean breakup.
Old wounds heal slowly, to help your ex get over you, don't hold onto any of their property, don't go places where you know they will be and don't respond to calls, or even insults directed your way.
Your new-found singleness and freedom is only available if you keep the breakup clean.
No "for old time's sakes" get togethers with your ex.
No contact beyond what is absolutely needed, if any.
Your own mental health depends on your strength of self and the true fact that the other person was smothering you, treating you badly, whatever happened before the breakup.
You become their healer, leave them alone more than they contact you and soon you will be out from under their controlling efforts.