I'm worried step-parenting is just not for me

[thesp]

DP and I have been together 3-4 years now.
He has 2 children from previous relationship, I have 1, and we have 1 together.

When we first moved in together, they slept over 1 night a week and came for tea once or twice once DP finished work.
We never got overly close but I didn't mind them being there and I enjoyed our family days out.

But things just kept getting gradually more difficult.
We started having them more days/nights and the more the time we had them increased, the more it became just a given that I'd have responsibility for them if DP was working.
They were raised differently to how I raise mine and the older they get, the more of a negative influence I feel they are on my DC.

Now when they're here, I just feel like the influence on my DC, the tidiness of my house and my time is just out of my control.
It starts making me feel like a guest in my own house and I just dread it. I get so overwhelmed when we have them.

I feel bad even writing it because I never confide in people about it as I feel so horrible but I'm just not even sure what to do. I try so hard every time and I just always end up drained and upset.

I'm not even sure what I'm asking, just for advice/support. The situation is just making me unhappy and I'm berating myself because I'm clearly just not a good step-parent and I wasn't cut out for it.

I dont get the impression that OP doesn't want to reciprocate. She would like to, but finds it very stressful particularly because of how messy her SCs are and that they are a bad influence to her own children. The probably don't have much respect for OPs standard or different ways of parenting and of course she isn't their mother, so imposing things on them is a lot more difficult. OP says she feels like a guest at her own house when they are there. It isn't that she doesn't want to help out, but this is affecting her wellbeing really.
OP I understand why you feel guilty, it does sound like your DP is easy going around the children. But your feelings are valid too and you shouldn't have to force yourself when it's causing you stress. You could maybe phrase it to your DP in an honest but gentle way. You could also say you find it hard looking after that many children and he needs to either adjust their visits (so more time with their mum if he works shifts) or find a new job. It's irrelevant how he feels because he obviously doesn't mind, whereas for you and your mental health it is a big thing.