First, it was my own fault, too. I thought that I was happily childless by choice for the first eleven or so years of our marriage, but I just went into denial when I realized my feelings were changing. I drank too much. I just couldn't face touching this third rail in our marriage, and when I finally did after I started menopause and became deeply depressed over my childlessness, it all blew up in my face. I felt so stupid.
I have to tell you, most men, but especially men who already have bio kids with a previous spouse, just cannot identify with your feelings. Please don't reproach yourself for not leaving, though I did the same thing. Leaving a spouse to pursue single motherhood is an awesomely hard thing to do. You are going to be OK. Once you are pg with your DE baby, I suspect your insecurity around DH's kids with his ex will abate.
Take care, and keep us updated,
Maggie (in VA)