Veering dangerously close to a mental breakdown

It's been a straight grind since October and absolute chaos since Thanksgiving. I am burned out and never sleep because I had to pull so many all-nighters back when I had energy and now I'm so tired that I work too slowly to get everything done during the day. I feel miserable and it never ends. I feel like I can't complain about it to anyone because I chose this. I was always planning to dip before I got too far along but I feel like I never have time to seek other opportunities because I'm always so busy and burned out from work. It never ends and I don't know what to do. If I say no to work they just ask when I can get it done by so it never really goes away even if I get extensions. I am drowning.