Compulsive liar

So my fiancée is a compulsive liar. He used to be addicted to drugs and tells me he lies because that's what drug addicts do. They're perfect liars. Well every time I catch him doing something wrong, he lies right to my face and insists. If I catch him doing anything that pisses me off he will never admit it. He just lies. I've broken down right in front of him many times begging him to just admit to his wrongs and he refuses. It takes months for him to admit to something he did wrong. I don't want to leave him because I love him to death and have a child with him. Any other ladies deal or dealt with this before? If so, what happened? How can I get him to tell me the truth without lying?? I've never confronted him about something and him straight up tell me the truth right away. I just can't live like this. I do everything for him and all I want is honesty and for him to stop lying. What can I do??