Yes, it is true.
And yes, it is the right thing to do, keeping some distance.
The more you let him in, the more vulnerable you are to him, the more he can hurt you. (Or hurt you through attacking things important to you like your kids.)
A romantic relationship starts out with you as total strangers at the beginning, and you don't trust strangers very much. Over time you take successively larger risks and make yourself successively more vulnerable to the other person. Hopefully you do this at a rate which seems safe to you, and the person proves that they are trustworthy at higher levels over time, as you let them in more.
It isn't safe for you to let him in farther. In fact, it may not be safe to leave him in as much as he is.
And yet, how can a relationship ever work if you can't do those things?
The more you shut him out, the more you limit your relationship. It may reach a point where the part which still feels "safe" with him isn't worth bothering with, and you will choose to end it.
OTOH, if you enforce good boundaries, you will stop participating in the messy games and patterns you have with him, and there will be an opportunity for him to do better. (After the extinction burst / adjustment period, that is!)
If you both improve, you may find yourself able to let him more.