You have freed yourself from a previous relationship where your needs were not fulfilled. You likely were cheated on, used, abused, and disrespected. You have entered a new relationship with someone who is nothing like your previous partner. This person adores you, is faithful, and accepts that you have a child with someone else. So does this make you happy? It seems, only for a brief moment.

What you see: How dare this person see anything have to do with my child as a sacrifice.
What you could see: I acknowledge that my having a child means this person makes sacrifices every day. I appreciate their willingness to make these sacrifices.

What you see: I am so disappointed this person doesn't do A,B,C, and D for my child.
What you could see: I am relieved for the ways this person helps me with what is ultimately my responsibilities.

What you see: For the sake of my child, I will put a strain on my current relationship, holding this new person accountable for everything because they are here and my ex is not.
What you could see: For the sake of my child, I will put a strain on the relationship I have with my ex, holding them accountable for having had this child with me, because they should be taking care of him/her and they are not.

What a difference it would make in the outlook a stepparent would have of their situation, if a bio-parent could just acknowledge the positives instead of dwelling on the negatives. A bio-parent would avoid so much misery by dwelling on the positive. Those negatives might even go away if so much time and effort weren't spent dwelling on them. To the bio-parents on the site, if it is appropriate to your situation, when you see your partner next or give them a call on the telephone and just kindly acknowledge they make sacrifices. Tell them you appreciate them. You may be surprised at the positive results you'll get for everyone in the household from two simple sentences.