"It was a rainy Wednesday in April, first period after lunch, and all I could think of was 'Oh God, this week is only half over'. I couldn't wait for the weekend. The school year had been hard, but the last few weeks shouldn't have been any harder than the rest, and I really didn't know why I was feeling quite as exhausted as I was. In fact, I had been pretty well constantly tired for the last few weeks, and I was about as engaged and motivated as my bottom set Year 10 students (ie not very engaged and not very motivated). Yesterday, one of my students had teased me about constantly checking my watch: I just wanted go home, even though I would probably do more work there; the lesson wasn't going well, but I didn't care. I was also asked why I yelled so much now (I hardly ever used to shout in class). I remember thinking that there is never enough time, and I just couldn't see the point if nothing was going to get done properly anyway. I felt like I was constantly falling behind and I didn't think I would every get back on track... and the weekend was still a million miles away."
Now, let's be clear about this: if you have read the paragraph above, and thought 'Wow, that's me. That's me right now.'; then our advice is very clear: your first step should be to phone your GP and make an appointment, or find some other way of getting professional medical advice.
The author of the paragraph above is showing classic signs of burnout. If you have just made an appointment at the doctors you might want to note that teachers are not the profession most likely to suffer from burnout – that dubious privilege goes to the GPs themselves. You can therefore expect expert advice.