I can say, as much as I love and respect my husband, if I'd known in advance how unrewarding and thankless and frustrating it is to be a stepmother, I would not have married him. I'd have married someone whose kids were already grown and out in the world, with the ex long out of the picture.
Truthfully, if I don't have a child of my own (bio or adopted), I don't know that I will stay married. I'm simply so resentful of all the time DH spent, um, c*ckblocking me on the kid issue, wasting, essentially, four of my last fertile years, only to have the one IVF I finally did at 42 fail miserably. We're now TTC naturally, with no luck so far, of course. He doesn't want to do DE, but we'll see if that tune changes if he knows I might leave. Without this situation resolving with a baby in my arms, I may not be able to stomach the continued Second Tier status.
My step kids like me, they are decent enough, and my DH worships me, but he doesn't feel any real urgency around kids, and I don't think he really gets how it makes me feel to just be his tag-along all the time. "Oh, they LOVE you!" he says. I believe they do, however, their loyalty is to their bio-parents, and frankly, I don't think that in the deep-down, overall familial sense, they give one sh*t about me. I think love and loyalty are very different things, and they don't always come as a package deal. To them, their mother walks on water. So be it.
I really feel your pain. It IS a pain in the ***, and often thankless, and you get NO credit whatsoever, only to watch "Saint Mommy" get all the applause, especially if she's a single mom now. (Never mind that DH might well have divorced her for very good reason). Also, my in-laws don't value me on the same level, because I don't bring grandkids into the picture.
Sorry I don't have advice. I'm pretty bitter about this myself. Just want to let you know I truly understand and sympathize.