I am a stepmum too...and I had all those feelings and more when I first met my DH and his kids were 1, 4 and 6.
It's now 19 years later and yes, it definitely gets easier! There are still plenty of dramas and arguments now they are adults with their own opinions and their mother is a poisonous manipulative liar, but that's another story...
My best advice to you is to stop fighting against it. I don't mean literally, I just mean give up on that huge internal struggle you are having. It is normal to feel resentful, awkward, out of sorts, anxious, unsettled when they're with you (quite apart from just having had your own child too, which I never had) - it will never go away completely in my own experience, so just roll with it - don't resist it or try to analyse it or try to make it go away, just feel those feelings, accept them and then let them go. As someone said...fake it till you make it!
I set my own mental rules for myself; his children ALWAYS came first, at the expense of my own feelings (not of your own child of course, that's different - you'll have to find a balance there!) I would never, ever, put my disappointment or fear or frustration before the well-being or welfare of his children, and I would never make him choose. Their relationship with their father was paramount and more important than mine with him, I would never stand in its way.
It's not over, we have a different set of problems now they're grown up, but it's not as difficult as it was when they were little. There is one slight downside to my backseat approach, and that's that we are not affectionate with one another. I am a bit of a cold fish, I'm not tactile and don't do physical contact, but with the children it was my fear of being rejected when they were small that prevented me from hugging, cuddling or kissing them (I did with the youngest when she was a toddler of course, but not once she didn't need carrying or dressing or soothing anymore). I never got over that and even now, there is only one of them I can hug without the fear of him pulling a face or moving away. And it's sad because now they're grown, and the eldest has some MH problems, I am really yearning to hold him and comfort him, but I think he'd die of shock. 
But that's ok. Their relationship with their father is the main thing, and they have that with or without me.
How you are going to handle this AND factor in your own child, I really don't know. You will need two sets of rules, and you're going to have to be exceedingly flexible! I wish you luck. 