Teaching IS a really stressful job. Everyone knows this and is well aware. I worked as a Teacher's Assistant for a kindergarten class and I was only there for about 4 hours a day, by the time my shift ended I honestly couldn't wait to get out of there and just unwind at home. I loved the kids I worked with, but it is a stressful environment. While I was there, we had one kindergarten teacher who worked in the next room quit because she was having trouble coping mentally with the stress, or so the teacher I was working under told me. She kind of scoffed at it, but I understood how someone could come to that decision.
Just know that you are appreciated at your work. Your students probably miss you. You are capable of it if you have gotten this far. If it is really something you want to keep doing, then perhaps you could talk to your doc about getting on anti-anxiety meds? You could also look into other positions that involve working with kids but don't require you to perform.