What I wish for those saying that you shouldn't have gotten together with your spouse if you didn't love the SC, or love them like a niece/nephew, is to recognize that sometimes we do all that and things change. And sometimes you need to detach a bit to survive and keep things harmonious. And "loving like a niece/nephew" doesn't work when you've been actively involved in the child's life from a young age, are invested in them and their second parent when they stay with you regularly over the years, and you have children that are their siblings.
I think it can be so hard for a stepmum, and things can change in a way they wouldn't with your own DC.
In my case, I loved and still love SDC but have had to mentally pull back because it hurts too much. The child I helped raise doesn't live with us full time and as they've gotten older, not even a teenager yet!! they don't want to come over because we have different rules than the lazy parenting of the ex. We don't let them stay in their room all day glued to a screen or eat and drink poorly. If we "make" SDC come over for their weekend and they don't want to, it's a downright miserable time, and their poor siblings, our DC, suffer immensely.
SDC has a whole other, different life away from us, and nothing prepares you for how that dynamic plays out as they grow older, especially when they are in that environment more than yours. Nothing prepares you for MIL/FIL treating your parenting choices like a joke because you're not the child's real mum, or treating dad's opinion as less-than because he doesn't have SDC full-time. Nothing prepares you for when this child, who is still a child, doesn't show any care for you, doesn't appreciate you, doesn't want to come round, talk while at their mum's, ignores their siblings, etc.
It's not that DH needs to do a better job - he reminds SDC, tries to teach the importance of family, etc. At the end of the day, you can't make them care if they don't. And realistically, you can't make your own biological kids care either, but at least they're stuck with you, at least they live with you full time, so there are still going to be even some small moments of tenderness, making up, just talking, spending time trying to figure things out, etc. You can also take your own kids for counselling but can't just freely do that with SDC unless their mum is on the same page.
Many people seem to expect a stepmum act like a real mum, love them like your own, but nothing prepares you for the disconnect of loving them like your own but having such little say because they aren't your own. If their real mum is in the picture and your views don't align, you can become so stressed at seeing how SDC is being shaped/influenced so negatively and you are helpless. This isn't a case of "accept them for who they are" when you know that's not who they are/were and it's a product of bad parenting that you can't undo. And you can try, but undoing bad habits and disrespectful attitudes is near impossible when you have them 2 days of the week every weekend or every other weekend.
When as a mum I say parenting is hard, people act understanding. When as a stepmum I say it's hard, people act like I'm the one doing a poor job. I have always gone out of my way to make SDC feel loved and welcomed, but in my case things changed for the worse and me, DH and the kids don't seem to exist to SDC when they're at their mum's. And it hurts more than anyone could know going into this stepparenting thing.