Boundaries are lacking and he may be codependent. Even if they had been together for 18 years, their kids are older and the two of them had decided to divorce for whatever reasons.
You are not unrealistic in your expectations. He has to have boundaries and he also has to respect your wish.
During your counseling, try to figure out what is allowing you to tolerate his behavior.
It sounds like he wants to have both relationships to fulfill whatever needs he has from each one.