Start by moving on. Don't engage your ex, delete Facebook, hit the gym. 

It begins by not tolerating disrespectful behavior. Draw a mental line in the sand over what's appropriate and what's not. If a person crosses this line, you don't tolerate it. You must stay consistent in this and stay faithful to this line. You are to never make exceptions to anyone for any reason for crossing this mental line. This line is your boundary and is necessary for protecting yourself against toxic and destructive behavior.