Hey! hope you're doing okay
I've actually been struggling with that myself this year but I'm in Year 12.. I ended up missing almost a term off school and was admitted to hospital twice, after a fair few emergency room visits - obviously not ideal when there's SACs and everything going on!
My advice would be to completely forget about exams for the moment - as you said, they don't count to your ATAR at all. The most important thing is to try your best to get better and start getting healthier to cope with Year 12.
I completely get where you're coming from with the lack of concentration - at my worst I could barely focus on holding a conversation with someone! You are in no way fit to be examined at the same standard as everyone else so I think it's more than fair that you sit these ones out.
My school have been really good about it this year, they let me drop a subject and were really flexible with my SACs, and I'm sure your school will be fine, especially since its just Year 11.
My ultimate advice is to just do your best to get well as soon as possible! It's so hard but trust me, you don't want this in year 12, and you don't want to end up in hospital. Take care xx