I hate being a step mom
What don't you like about being a step mom?
I have to take care of my husband's kids 24/7. I feel like I'm always on call.
Does your husband know how you feel about his children? Treating step kids as anything less than your own children can definitely damage your relationship with everyone involved. Talk to your husband about a compromise, where you are still around, but not needed around the clock.
I'm too stressed and it's not fair to have to take care of them.
I understand that it's tiring to take care of children so much, and that's why you need to talk to your husband and tell him you are drained because of it.