Those threads always make me sad because I have been on the receiving end of this.
My mom had me really young (16/17), my father was abusive, she had my brother at 18 and those years were a lot about survival for her (which I acknowledge)
She then found a new partner, had two kids and is now married to a third person and just had a baby.
It was clear to me that as soon as my step-siblings were born, our mom sidelined me and my brother, but me specifically for having been the first one and the one who I know, somehow trapped her.
I know as much as she will refuse to admit she hates seeing some our father in us, she probably also recent the fact that having children meant she had to stay longer in an abusive relationship than necessary.
Interestingly enough, she has done the exact same thing with my step-siblings, she split up with their date a few years ago and as soon as she found a new partner, sidelined them also, I haven't met the new baby so I have no idea how she feels about her vs the rest of my siblings but I know my (step) siblings feel sidelined as well.
Personally me and my mom are no contact because of this (hence why I haven't met the baby), I don't begrudge her feeling the way she does about her situation, it can't have been easy and I could understand being resentful but it's so incredibly unfair to make your kid pay for decision you made (not using protection/choosing not to have an abortion/ choosing not to give up the kid for adoption). I get that my mom suffered and that as such I am a constant reminder of her trauma, but imagine what it's like, having a deadbeat father and a mother who begrudge your existence for circumstances you didn't chose and are affecting you as much as her.
It's traumatic and it's definitely impacted many aspects of my life.
I have no idea what the solution is because I obviously haven't found but I guess it's good you are at least acknowledging it.
Do you think your feelings will change if you were to separate with your current partner and things went sour?
I feel like my mother was/is in the quest for a perfect happy ending and therefore remarries and have children on a whim to try and ''erase the past'' and restart history, and I am probably very petty and bitter but a low-key part of me hopes that this new baby brings her the same feelings I have given her and that they will have a similar relationship I do with her, because while I don't want this baby to suffer in any way, the behavior of my mum has been so painful that the idea that she could be the perfect mum with the perfect relationship to this new little girl (all my other sibling/step-siblings are boys so it's different) and act like I was just a bad moment of her life to get to her happy ending with the little girl she had always dreamed of will no doubt, cut too deep.
Again, not sure what's the solution is, obviously these types of feelings are tricky on both sides of the balance, maybe working on it with a professional could help?
I wish my mom had acknowledged it and tackled it, her not doing so has made me stronger but also ruined many aspects of my life I wish hadn't been affected. Remember that your son is also a victim of the circumstances surrounded his birth and that of all the protagonists in that story he was the one with the least say in it.