Empty Chair

The empty chair technique is a gestalt therapy technique in which the client engages in a role-played conversation with an imagined person. The patient sits facing an empty chair, and imagines that a family member or some other specific person is sitting there. The patient will talk to the person, either to verbalize things they for some reason would rather not say in real life, or to rehearse future encounters with that person. A real person (or even the patient himself) can also play the role of the individual in the chair.

This is a good technique for someone who is naturally timid and has a difficult time being assertive.
A good scripture for assertiveness in relationships is the following;

Luke 17:3
‘Be careful how you think. If your brother does something wrong, talk to him about it. Your brother may say, “I am sorry for what I did. I will not do it again.” If he says that, you must forgive him.

Sometimes when you're assertive the person/people you're dealing with are determined to find fault with you and justify their position. In those times the best course of action is to walk away and remind yourself daily that you did the best you could do and that you are not responsible for other peoples decisions to refuse normal healthy relationships.