Moms of 3+ babies, burnout *LONG*

I feel so exhausted I have 3 children and pregnant with my fourth at 20 weeks. I feel ashamed for saying this but I am burnt out I do not get a day off from being a mother ,self-care is a joke. I work at the school my oldest 2 go too and I have to do everything for everyone with a smile on my face. I want to cry I am so depressed I don't even know who I am. I want a day not weeks or whatever without my kids. Just so I can find out what I want to do. I teach, I am a full time student, I have a fiancee who i do pretty much everything for, and a dog. I have literally lost my mind sometimes I can get down right mean and I have said something no one helps out at all I can't even get a shower by myself. When the kids sleep I sleep because I have too. I am so upset that I even feel this way but I want to run away to get a bit of alone time and my kids are 5 almost 6 and under. Like they are young and I know I had them i take care of them but i am massively depressed and i am going to have another in june i just want to lose it. I know it's not the end of the world but I have most of my conversations with children I don't have any time to myself and I literally feel like a shell of myself I don't get out fuck I don't even have friends. What do I do here? Leaving my kids with their dad isn't an option bc he wont watch them all so I can do anything even at home. I feel used up by everyone around me and I am going insane. I am only 24 i am young and i feel like my life is better off dead because I am really no one important.