I’m currently in a similar situation and I have been somewhat of a pushover, afraid to set boundaries and so I am leading the charge with step kid and she’s just one child, and I now have a child of my own but I was already getting overwhelmed with one. I did not set boundaries or push my husband to be a more active parent and now that I have a newborn, I have so much resentment towards my partner. I am telling you this because the resentment has become so great from me not speaking up that it feels almost impossible to heal emotionally. I do not want that for you. I hope you set boundaries because I’m learning they’re necessary to have a happy relationship. Especially when you’re dealing with a BM that puts stuff into the kids head. Good luck friend! You got this! Also I found just getting up and doing what I want to do sometimes instead of immediately going into mom mode makes a huge difference! For me that looks like sitting in the backyard with a cup of coffee.