Your last line, the question about dating a guy still in contact with their ex really struck me. You know, if you're dating just a guy then I can see that thought, even agree with that thought. When talking about a FATHER, well, now, that's a whole other story and should be phrases as, "Who would date a FATHER who doesn't keep contact with the mother of his children?"
I mean no harshness when I say this: You're being childishly selfish. Why do you think that is??
Here's the thing. If you have married a good man and a good FATHER you should be walking around like a proud peacock. Men who are good FATHERS don't just pick any old pretty face, they are looking for GOOD MOTHERING material, they're looking for an amazing woman...you may not see that when you look in the mirror, but HE DID! Think about that. If you don't think it's there, CREATE IT.
STOP punishing your man! Stop it right now. And STOP punishing YOURSELF for whatever it is that makes you unlovable. You have four hearts beating for you, that love you, and are looking to you for security...FOUR. You don't have time for ugly voices in your head to detour you from the Bliss that awaits you in just settling into your life with you MAN and children.
It's as simple as this: BE the soft place for him to fall, be the comfort he needs, be the salve for his wounds, be supportive, be cute, be sweet, be fun...and he's YOURS!!
Another thing I noticed in your message is that you seem to separate yourself from the parenting things...parent/teacher conference or swapping children leaving them isolated together...Honey, there may be STEP before PARENT but the important part is that you are indeed a PARENT and you need to include yourself. There's no reason for them to have that exclusive together, no reason at all...GO WITH HIM every time you can!! As a stepmother you have taken on the responsibility to look out for your stepchildrens' best interest, to be supportive of them, and to be their ally...you need to be apart of the school meetings and in receiving them from the care of their mother.
If you can be civil with each other, why don't you take it upon yourself to text the mother for information FOR your Man???
The trick of being a happily married is being pleasant and creative. If there's a relationship you're not too keen on, the BEST way to handle it is to make it obsolete. If she'll allow you to communicate with about pick up times, or little things like that, take it and run with it. They'll still have to talk about important things but the more you PLEASANTLY involve yourself, the less you have to allow that imagination of yours to take your marriage from you.
As a stepmom and a woman with and ex husband whom I've had children with, let me ASSURE you that if your husband things about being married to that other person it's NOT in a whimsical longing way but in a "how the heck did I ever marry that one?!!" kind of way. I often wonder what in the world drove me to think it was a good idea to marry that first male person and when comparing the two husbands I thank my father in heaven for giving me a real MAN to love and be loved by. If you're a good woman, pleasant, helpful, fun and safe for your man, I promise he's doing the same!