I felt like this until I put some boundaries in place.
My now DH’s kids were 8 & 5 when I first met them. They were 11 & 8 when we moved in together.
At first they were on their best behaviour but over time their behaviour (especially the older girls) deteriorated. They couldn’t do anything for themselves, were fussy eaters, would leave wrappers & drinks wherever they dropped them and started to be unpleasant around (the older ignoring me, not speaking to me, eye rolling and generally passive aggressive). When I was ignored and started getting one word monosyllabic answers - I knew to save my sanity I had to make serious changes.
I stopped doing anything for them - cooking, cleaning, washing. Not my kids - their dad can do it. No thanks from them? No thank you.
I stopped going on days out / activities with them. They had become such hard work as I was ignored and never a word of thanks. Best is they spend 1-on-1 time with dad.
The older (14 at the time) - if she wanted to eat beans on toast with her hands. Let her be. Not my kids, not my problem.
When the kids came - I would do my own things.
Read up on the NACHO method of step parenting. Once you take the pressure off yourself and realise they aren’t your kids, not your problem, the relief is immense.
My advice - treat others how you would like to be treated yourself and stick to your boundaries.