any other step moms?

My husband and I got married 8 months ago and he has twin 10 year old sons that we have the majority of the time. I'm exhausted. Emotionally, physically, this is so much harder than I thought and I feel like a failure. I'm failing him, I'm failing as a step mom. And I always thought I would be an amazing mother. But I get easily frustrated when I see them manipulate their dad and disobey. They are generally sweet boys. But the transition days between their mom and here are so very hard. Any tips?