I feel the same way, we're not married so I'm not officially step mum but we live together and have a 6 month old. My partners daughter spends the time 50/50 with us and her mum but my partner is the primary parent, main contact for doctors/school etc, he likes to be in control as doesn't fully trust her mum.
Since having our baby I do feel a constant sadness, the 5yr old demands so much attention from her dad when together, we have argued about it so many times, he wants me to focus on her too but I don't have the natural urge anymore because I want to solely focus on bringing up my baby. She is a very good 5yr old though, never complains, he does have the typical dad guilt and lets her do anything she wants etc / I try and gain some authority and she respects me but then he'll get annoyed if I deny her something, like chocolate for example (if she's already had a load!).
He often wants to collect her early from her mum on the weekends she's supposed to be there and when I say I'd rather he didn't because it's our weekend, his time to spend with me and our son, I'm made to feel guilty and am accused of being jealous.
It's so so hard and it's difficult when I know no one else in the same situation as me. I love him so much but I worry our communication has got quite bad as I can feel neither of us can say what we really, really want or how we really feel due to being worried about the others reaction.
Just saddens me that he doesn't seem to be excited about the prospect of us spending time together as a 3 when it's so important to me. The fact that he has shared something so precious and special with someone else makes me feel sick to the stomach, they still send photos to each other which I don't seem necessary. I didn't think I was going to feel this way at all, it all changed when the baby arrived and I felt the unconditional love and happiness for this being, I just want him to have all the special time with his dad and she did and I hate the fact he is treated as a 2nd child when he is my first.
That's a proper rant, I apologise x