Sorry, this may sound harsh, but I think you need to try and stop feeling sorry for yourself. When you have kids, there is no reason why your partner won't be excited about them. He'll see them every day for a start, and that in itself will be much better for him.
I think you need to give yourself a good kick up the bum, and get on with enjoying the time with your DP and SDCs. As Christmaspud says, look at the positive. If you keep this resentment going, I can imagine your DP's post on here:
"My DP doesn't want to do anything with my children, she thinks they're going to get everything "first", before any children we may have together. I know I'll love my children with her just as much as the children I have already, but how do I get her to see that?"
If you don't your resentment will drive a wedge between you and your DP, and you may not have those children to worry about.
If you feel that robbed taking your SDCs to the park and having baths, then IMO you need to find yourself a man who has no kids. Sorry.
I speak as a SM to 2 DSCs, so I am definitely NOT one of the "you made your bed now lie in it" brigade. I understand how hard and difficult step parenting can be, but you need to decide if you can get over this resentment.