I’m so sorry, I can’t imagine having to teach in those conditions. Teaching under any conditions is hard enough as it is, it’s got to exceptionally difficult the way you are feeling.

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I can’t remember, but were you on some kind of short term disability when you initially left work? Did your pdoc approve you to go back to work, or did you just have to go back to work for financial reasons? I remember you were worried about going back to work, and I hate that it’s as bad as you expected. I don’t think it’s all in your head, unless “in your head” refers to the stress, anxiety, lack of concentration, memory issues, that affects many of us. And makes it hard for some of us to get through a day at work.
Do you have any anxiety meds you could take each day, just to get through the school day? Or something more relaxing you can do right before the school day since your anxiety gets started at night? Like get up 15 minutes earlier and do 15 minutes of mindfulness or meditation techniques to get you started in the day? They have a few apps for that, I haven’t used them yet though. But I did a mindfulness activity with a social worker during my last hospital stay, and I was surprised how calm it made me feel for quite a while. If you can spare just 1 to 2 minutes of privacy at school (hard for a teacher, I know) there are even calming techniques you can do with just 1 or 2 minutes of time. Ask the co-teacher to monitor the students for just a few minutes, and go do some calming techniques. If she gets an attitude, tell your principal you need these moments to get through the day. Ask for your own 504 plan.

. You really do need some help to get through the day!
I don’t know if you’ve exhausted all your options, but I used to be a state government employee (not a teacher, but did developmental testing) and I realized all the money taken from my paycheck each month and put towards our retirement system allowed me to take advantage of many benefits because of my condition. I don’t know if you’ve already looked into that or not with your state. Just hang in there.

I know you want to get through the year, but I hope there is some option for you if it’s just too much. You do not deserve to feel like that everyday.