How to navigate the feelings that come with being a step-parent?

I just wanted to get this thought out of my brain by posting it here.

I am a 32f who doesn't have any bio-kids, the past year I have met the most wonderful man who comes with some lovely kiddos, so I have been navigating the world of step-parenting (I don't consider myself a step-parent yet, but the kids do).

The thought that crossed my mind today, I absolutely love and adore the kids in my life whether that is my nieces and nephews, my best friend's kids, or kids from close family friends, all of them. I love that I can shower them with love and affection, spend lots of time with them and be included in their lives, and sometimes help guide them in life. I am that fun Aunty that they get so excited to see and want to spend time with. I am also excited about the thought of having my own biological kid one day, and get to experience life as a parent.

I also adore my partner's kids. However, coming into the role of a step parent, I feel that it will not be as easily accepted if I form the above relationship with my partner's children. It's like this title of "step-parent" immediately puts this divide between myself and my partner's kids. As in, they are not my children, I am not related to them in any way, so I am not supposed to form this bond or affection for them. It kills me, because I hope to be that same "Aunty" type role in their lives, I am in no way looking to be a mother figure to them. But I can't help but think, am I just seen to people as this extra person off to the side in their lives who is in a relationship with their Dad?

I'll use an example, when my partner's daughter held my hand for the first time (she initiated), I freaked out internally. All I could think was, is this appropriate? Am I doing something wrong? Have I crossed a boundary? But if my close family friend's kid held my hand, I would not second guess it.

My partner is encouraging of developing whatever type of relationship with them as I or the kids are comfortable with. I am not expecting to have a 100% always sunshine and roses type of relationship with them either.

BM is not very nice to me, and pretty much devalues any happiness or excitement I have with her kids. For example, she once said the only reason that her daughter is excited to have me in her life, is because her daughter's friend at school has a step-mum and she just wanted one too.

Maybe its because I read the stories on this page often, and a lot are negative so I'm teaching myself these things about what a step-parent is. Maybe its the worry that the as the kids get older they'll dislike me more, or that their mum will be in their ear about me and it will rub off. Maybe its just an internal struggle that I have to navigate my way out of? It is so confusing to feel like this.

How do you all navigate these feelings if you have them?