You left the relationship, or the relationship left you, but you can't stop feeling resentful, frustrated, angry, fearful, sad and jsut not yourself around your ex. Whether you are divorced and co-parenting, sharing a town or community or simply can't stay out of each others lives, there is no reason why you can't get yourself to feel, at the very least less awful around this person. 

Just because the relationship ended doesn't mean the triggers stopped. Time to learn to take responsibility for your role in how you feel.