fighting about my stepson is going to rip my marriage apart.. HELP

He has a 7 year old son who is with us sat, sun, mon, tues, Wednesday morning one week. Sun, mon, tues, Wednesday morning the next. I have loved his son since day one. But lately his behavior has been out of control. Constant disrespect and not listening. Me having to tell him things a thousand times in a row. Then his dad gets home and he acts even worse(fighting for attention) Me being pregnant right now.. I can't handle this stress. I watch this kid every Sunday while my husband is at work. Monday, Tuesday, Wednesday I take him to school in the mornings because my husband is at work. If I get out of work early and my husband is still working I go and pick his son up from school on our days and take him home to do homework and wait for his father to get home. My husband sees the way his son acts towards me. And while I'm not asking for him to get rid of his son, or beat him until he listens. But I do expect him to punish his son for disrespecting me and not listening to me while I am watching him. My husband recently made the comment that I don't do anything for him and that I "certainly treat him like a step son" .. but yet I revolve my life around his kid that isn't mine. So that he can see his kid more than once every two weeks. I am to the point where I am so stressed out I am having anxiety attacks everyday. Being 5 months pregnant.. this can not be happening. I don't know what to do. But it's like my husband expects me to do everything for his son. I'm sorry, but his mother is in his life and I'm not the one that needs to be doing all of this. 

I feel like I involved myself too much and now I am in a hole I can't get out of. If I stop watching his son.. I'm an asshole. If I kee watching him and disciplining him.. I'm an asshole. My husband isn't budging on the topic. And thinks I should just leave him if I don't want to watch his son anymore. And that's not it. I just don't want to do it all the time. And I shouldn't be obligated to do these things just because I'm his stepmom. 

I'm at the point of feeling like a free babysitter who never gets a thank you. Feeling hopeless, and just want to save my marriage. But my husband hates me right now. 😞😞