@HebburnPokemon you asked up top for advice on how to manage your resentment.
First off - I am very sorry for your loss of the children's other parent (whether they were your spouse or you'd split up). I also note you've used fairly gender neutral terms so am not assuming you or OH are a woman /man couple.
You have said a few things that jumped out at me.
- you say you are "an introvert"
-your children are also "introverts", "docile" and so on.
- your step kids are rowdy and make a mess and create chaos
- you feel invisible and sidelined
-you feel resentment - which is anger, boiled down and not able to be expressed
- you sort of think you have to suck it up because it's fair that you live with someone with children.
Unpacking this gives you somewhere to start. Could you have your own therapy?
I'd ask myself questions in three big areas.
Are you angry that you don't have the life you originally chose, with your own children, in your own peaceful home? It was taken from you by death. That is awful. Is your resentment really this anger, that you lost control of your life - but you can't express it, as it isn't the other children's fault, so it comes out as resentment for their different ways? If you get support for your legitimate grief and anger might things feel different?
What do you have invested in you being introverted and your children being the same? Is this a part of your identity that you value? It sounds like it might be. Do you feel instinctively that to be a good person is to be introverted and docile, quietly observant of others and meeting their needs without them having to ask? Maybe the children just aren't like you. (And is DP the same?) Could there be another way to see their behaviour, if it's just rowdiness? As love, expressed loudly? As them being comfortable? Might be easier to live with.
Again, as an "introvert" are you perhaps not good at articulating your needs? How do you articulate what you need to DP and the other children? Are you good at telling the difference between people living differently from you (mess and chaos) and people being disrespectful (fighting, rudeness, breaking things). Have you had a conversation with DP about what they think the boundaries are? Is it actually just painful that the step children are allowed to express their needs loudly and without making themselves small... have you ever been allowed to do that?
And all that jazz.