How to professionally resign a long term sub position

I am currently long-term subbing and I absolutely hate it. I go home every day crying and stressing about going back the next day. I want to finish out December (the position is supposed to go to end of January). But how do I professionally tell them I’m not coming back after Christmas break? I don’t like using mental health as a reason or excuse but genuinely this has opened my eyes. I don’t think I want to be a teacher anymore after this.