Firstly, can I say how sorry I am, that you lost your little girl. That?s so incredibly sad. It?s wonderful that you?ve been blessed with a baby boy, but I doubt that lessens the pain of your earlier loss?
Have you sat down with your partner, and told him EXACTLY how you feel? I sometimes think men can be incredibly slow on the uptake; and unless you spell things out very clearly indeed, they don?t always pick up on issues. You say that your DP always invites his daughter along when you?d rather he didn?t: two things, I?m surprised she wants to tag along at her age, and also ? is your DP absolutely clear on how you feel about this?
Of course the children from his first marriage are important, but his relationship with you has to be given at least equal priority in his life, not to mention the child you?ve had together. Those precious first years with your little boy can never be repeated, and I?m not surprised you?re starting to feel resentful.
The combination of a terrible loss, followed by a new arrival, then back to work, and a house-full of teenagers (two of which are step children) sounds overwhelming.
I?m pleased you?ve decided to post here.