Feeling used up

So I had my fourth baby mid June and now I feel so wore out. My oldest is 6 If that gives any context. So i am doing everything around the house cooking cleaning laundry. My fiance doesn't do anything. I now have a job, go to college, now next week I will be my oldest childs teacher too. And all he does is complain to me! Not to mention I breast feed my 2 month old. I am so wore out all the time I feel like I'm a prisoner. He only wants sex from me and somedays I really dont feel up to it. Today is 2 days in a row he hasn't had it and I was cleaning the house he calls me into finish him. How is this shit fair? I know sex is important in a relationship but I feel like a slave to everyone I live with. I just want to rant right now. I'm just super upset because no one seems to ever see my side of things. I know kids will be kids too. Just sometimes it gets tiring when you are the only one doing anything for them, it would be different if I was a single mom but I'm not. I just want some help, and not to always feel pressured into sex because it feels more like an obligation that way. I'm so exhausted all of the time it is literally the last thing on my mind.