@NewNameNigel
A man who clearly states his priorities and positions up front is an honest man and that behavior is refreshing in a world where so many men and women attempt to lie and mislead their partners.
The OP is not a stepmother. She may aspire to be a stepmother, but she does not live with this man. His children have 2 obviously engaged and committed parents. No where does she suggest that this man has any expectations of her regarding his children. She describes a man who is seemingly a very competent father and one who is actively co -parenting with his ex.
I am sorry, but I did not see a step issue at all.
Of course it can be painful to not be able to have what we want. That is true for most people regardless of relationship status.
The OP found an attractive, engaged man who is not offering what she desires in a partner. He is not misleading her.
He has told her and demonstrated who he is and on what he intends to focus. He has been honest and she may wish that things were different, but she cannot fault or blame him for being who he is.
She is holding on to him while tormenting herself, but that is her choice. He may simply want loving companionship. He has children and no desire to be married. Obviously, that is not enough for the OP, and by staying with him, she is losing the opportunity to find someone who might share her outlook and goals.
We cannot blame someone for not sharing our dreams. Each of us should be free to fashion our life around our own personal goals and dreams. We are incredibly lucky when or if we find someone who wants the same things. The OP has not yet found that person.