I hear what you are saying. I was "the other partner". My spouse has two children and we recently had her nephew move in with us. It was difficult for me to adjust to going from "single" to family, although I really wanted kids. We have been together for 6 years now and married for one. The children were 4 and 6 when we met. We now live together, but did not for the first 5 years as we needed this time to "adjust". My partner and I still have some conflict, but at the same time, it is about the kids as number one. From the "other" side, as much as I wanted kids, it was hard to transition into my new life. Birth parents have the advantage of "growing" with their kids. Us step-parents sometimes take some time to adjust. However, I wouldn't have my life any other way.
Loving my life,
Tami F