To hand in notice after 8 weeks?

[MiniMumMax]

Secondary school teacher. V experienced in private. Always had great feedback from parents and students. Always loved teaching.
Decided to move to mainstream. Had worked in mainstream before but not for a while.
I feel like I’m failing everyone, like I can’t get the students to pay enough attention that their marks are rubbish. And they’ve brought me in as an experienced teacher who is supposed to be really good. I feel really guilty all the time that I’m not good enough.
It’s giving me such bad anxiety that I can’t sleep/eat properly and the planning/admin is using all my free time.
Staff are lovely but I’ve not cried on anyone and told them this as I feel like they employed someone capable and they’ve been shortchanged.
The AIBU is - should I just leave? Yes - YANBU - try something else
no - YABU - you should stick with it and see if it gets better.
And if I leave. What do I do?

I left teaching because I wanted to make a difference. Unfortunately the reality of mainstream is your there to get ticks in boxes and meet targets. Mainstream education doesn't allow for the students as humans anymore.

Firstly, please don't beat yourself up @MiniMumMax. I bet you are doing a great job and it's just a period of adjustment.
I'm a teacher in a state primary, following a career change after twenty odd years in the private sector (different industry). The first year was 'interesting', but by the second I felt I had really found my feet and the challenges were refreshing rather than overwhelming. Please try to think of it that way ... it is all new to you and you need to be kind to yourself while you get used to it.
It sounds like you really care about the kids, and that, frankly, is the most important thing. Good luck and take care.