There are so many variables involved in step-parenting relationships that it's a unique experience for every family.
I know families where the (now adult) child and step-parent have a stronger bond than the child does with their bio parent. This normally reflects the age of the child when the family first comes together though, combined with a real effort being made by the step-parent and the conspicuous absence or general lack of consistent involvement of the non-resident bio parent. That said, one of my close friends has a beautiful relationship with her step-father and he wasn't a part of her life until she was 14 (she's now 50). The kindness and support he gave her at a very vulnerable time in her life, along with a real gelling of personalities, has made that relationship work long beyond the duration of her step-father's relationship with her mother, which ended years ago.
I've also seen step-parents moved in with families with no real thought about what that means for everyone long term, with both adults only thinking about the fact that they're in love and want to be together. Rarely works.
DH is step-parent to my DC. We were together for 6 months before he met the kids and another 6 months after that before he stayed over. It was a very, very slow process of integration. I told him what I expected from him in terms of being a step-parent and acknowledged that it was such a big ask it was arguably unreasonable. However, I also made it clear my DC were my first priority and deserved no less so if he wasn't prepared to step up we would continue to live separately or split up. We discussed finances etc long before he moved in and when we got married our wills were carefully crafted to protect everyone's interests.
DH has been a part of their lives for longer than he's not now. He's the one who's stood in the rain (often after only 3 hours of sleep following a shift) cheering them on at sport's matches. He's the one who attended school events either with me or instead of me when my job has got in the way. He's the one who set up an extra savings account for them because he wanted to see them have the best start in adult life. My DC do not call him dad and our family set up is not a secret but only my DC's friends know he's not their bio father. They generally refer to him as their father or us as their parents. They have never thrown the "you're not my father" line at him and they're grown up now so not likely to.
If you were to ask DH about his perspective, I'm pretty sure he'd tell you he had no real idea what he was taking on and at times felt it was a pretty thankless task. I'm not sure if he'd admit to being resentful of the limitations being a stepfather placed on him but even if he felt it he was mature enough to realise it wasn't the DC's fault and he had made an adult's choice to be in that situation. He's never, ever made us feel that we should be grateful that he took us on 🤮. He says being a stepfather has made him a more rounded person and he genuinely loves family life (sometimes more than I do I think).
I think for us that there were some unique to us factors that helped. Firstly bio dad not being in the picture at all (his choice). Secondly, DH always wanting children of his own but it never happening for him (we've never had any DC together as we both agreed we were too old). Thirdly, the power balance in our relationship is very very equal, not just in terms of earnings but domestic chores. DC also pull their weight. It sets a fundamental base line of respect IMO.
Statistically, step parents are more likely to abuse the children in their family than bio parents. Many step-parents just don't have a strong bond with their step DC and IMO nobody has any real right to demand that they do. It's a bloody tall order and many bio parents fail at it! However, don't become a blended family in that case is my view! Blood is not always thicker than water. Many step-parenting relationships are wonderful!
OP, I can't tell from your post if you're genuinely unhappy with the family set up full stop or just blowing off steam because you've just dealt with something. I think it's fine to say it's hard and thankless at times - that's parenting generally! You've clearly got the maturity to recognise it's not the DC's fault, so you're probably doing way better than you think you are. To try to make things better, try answering the following three questions. 1) How do you feel about your SDC? 2) How are you and your DH sharing parenting and household responsibilities, including finances? 3) What do you do with your SDC that is fun and will help create a bond?
The last one is key. It's the bond I have with my own DC, based on shared experiences, which helps me put up with the drudgery of parenthood and I think DH would say the same.
Hope it feels better soon . 