Your stronger than you think...I agree with just not telling her when he's coming over..or if he tells her then don't show.. depending on her age, maybe just say that's the way he is an leave it at that. How ever you decide to handle it just make sure she knows how much you love her an that his not showing up is no way her fault. My grandkids Dad only sees them maybe every 6 or 7 weeks he will call at the time he's suppose to be here an say he's not coming ( he's not an addict just a jerk), but the oldest is 7 an she is starting to figure it out. We just told them that some Dad's like to spend lots of time with their kids an some don't, an that they happened to get one that don't.
Will be praying for you an her.