oh yes, i know where you're coming from!
I have been with my DH for almost four years, living all together with him and the two DSCs for 1.5. We have a DD (2) as well. In our case their mum is deceased. So like you I am a stepmother 'fulltime' - not that all stepmums are not fulltime in one sense - but I mean that the DSCs are here 100% of the time and there is no 'other parent' to take up some of the responsibility.
I had one kind of life on my own, and then another with DD at the start of her life - I was happy as we were and coping very well, and making my own choices.
And then we all moved in together and I became responsible for a family of five - my life totally changed - it all felt very sudden and shocking. DH's life did not change nearly as much, in a way - if anything, his got easier, while mine got much, much harder.
I am unhappy a lot of the time. It is a struggle. I suggest you do get counselling, to figure out what you want and also maybe to help you see if there are strategies to help you cope. For me, I have committed to this - I do not want to give up our family. I love the DSCs and I love my DH so I want to make it work. But that means, for me anyway, figuring out some ways to still get what I need and especially to get what I want for DD - I don't want her to lose out because we have to make so many choices based on what the DSCs need or want - it is one thing for me to sacrifice and compromise, but I don't want to sacrifice and compromise on her behalf, IYSWIM.
So, no answers here, but loads of sympathy.