Coly, I am sorry you are here! It's so, so devastating!
I agree with the others. And SH gives sound advice and solid support.
So I am going to come at you with some 2*4s now. Why? Because I want to help you. So I am going to tell you what I needed to do, but didn't, and so therefore it took me 10 months to LET HIM GO.
I have been reading here for several years, but only started posting a few months ago. Your story very much reads to me that your H is having an affair, that he is wanting to pursue this other option, but that he may be keeping you as plan B. In the mean time your sitch is complicated because D is his step-D and so you must play a role in their R.
I think you should go Dark. Do a 180 and shock him, no contact, no calls, no texting, and show NO INTEREST in him whatsoever. You are hurting and scared so you are holding onto whatever he might be offering. So think about if this was a friend, and what would you tell her? H just picks up and leaves her and her daughter and selfishly tells her to wait around until he makes up his mind. Uh, no. HECK NO!
You have every right to be hurt and angry! But right now he is not a safe person to share any of your thoughts and feelings with. He doesn't get anything from you right now. No contact, no pursuing, and no family time! NONE. He walked out and so until he can feel that loss, and really feel it over some time, he most likely will not reconsider.
If and when he changes, you will know. Your instincts will be right, just like they are right now about his behavior. Until then you must let him go, you GAL and be mysterious and become that fabulous woman only a fool would leave. Not to win him back but because you ARE fabulous and you deserve a man that appreciates you! If he notices, if he starts temp checking, or whatever, you simply continue on your path. Until he is ready to do his own 180, then do not go for those crumbs. Simply say "you have given me a lot to think about and I have some choices to make too." End conversation. Exit. Do not go for the crumbs! He needs to FEEL the loss.
In terms of your D, well she is the victim here. But it is up to him to work on an R with him. I would say let him talk to her and see her as much as they both want. But not as family time. And you do not initiate ANY of the planning. When he does see her, you make other, fun plans, and exit as fast as you can. No time for this guy who bailed on you, my siree.
He may notice right away, or he may not, but over time he will realize what a fool he is! So just pretend right now that he is the nerd in HS that drooled over you every day and did all your HW. Nice kid. No reason to be rude or ignore. But really not worth much energy at all. The nerd des rued you but knew he couldn't have you; this is where H needs to fall now. When he leaves you can cry in your pillow or phone a friend.
Starting right now, you will fake it until yiu make it! And let yourself enjoy some times supportive people and also give your poor mind a rest from this.
You got this.
Blu
“Forgiveness liberates the soul. It removes fear. That is why it is such a powerful weapon.” – Nelson Mandela