I would also guess that in the beginning, when the children reflected any joy about time with dad you quickly quashed it because of your anger.
In other words your children have learned to tell you bad stuff about dad/girlfriend cause they think that is what you want to hear. Children pick up quickly on these things and quickly learn what to say.
You need to seek counseling for you and your children. One of the reasons people probably are not doing much is because the kids tell other people different stuff. If at all possible you should invite the Stepmom and dad as well. It won't stop until the adults work together.
It is also probably another reason middle one is depressed. it is very stressful living this way.