I read your post and I feel like I can totally relate to how you're feeling. Sometimes, I feel the same way. My DSS is just a little bit over 3 years old, and as sweet as he can be, he is also very spoilt with attention, which frustrates me. I cannot be affectionate towards my OH, without his son moaning that it's his daddy and trying to get his attention. I understand this is completely normal for a child his age, just like the behaviour you describe from your step children, however it is also frustrating. I find that a small part of me cannot sympathise with my DSS, when he's having tantrums, because as much as I love him, I didn't give birth to him or didn't raise him from day one, so I definitely act a lot stricter and can see clearly past the tantrums, whereas my OH can't, and always tells me I will feel differently when I have my own children. Does that make any sense?
It was a lot easier before we lived together, but we have lived together for a little over a year now, and I am pregnant with his child (12 weeks), so now I feel like I can have a say in raising this little person, whereas before I tried not to get involved in my partners way of raising him, because he is an incredible dad and I didn't want to undermine him.
For example, today I tidied up the bedroom, which took me a while, and I found a lot of bits and bobs of DSS toys. Under the bed, sofa, in my washing basket, in the wardrobe. My DSS makes a massive mess all over the flat and I have always had an issue with it, but as he's only with us for two nights, I let that slide. However, today I spoke to my OH and said I think we should make a rule, and that will also include our future bean, no children in the bedroom. I explained that they can come to cuddle and see us obviously, if we are spending time there, but not bring their toys and make a mess of the entire house. Our bedroom is for adults, and as DSS has his own bedroom, and uses the front room as his playground, I think it's a fair deal. My OH agreed with me, and I am allowed to ask my DSS nicely to take his toys back to where he can play with them.
To conclude, I think it's important to have certain rules and know when to speak up. Sometimes, when I feel like I'm being a little bit too much, I just walk away from the situation and do my own thing until I calm down. Yes, children are a handful, but we need to remember that we want to create a childhood that we always dreamt of for ourselves!
Also - Don't be so hard on yourself. I'm sure you're doing a great job!