Don’t sweat your mental state, lots of us in the same boat. Second time for me in 3 years. Even though I was expecting it, and didn’t really like my new position, anyway, it very stressful. You’re in fight or flight mode all day and nothing you can do about it. Just recognize that it’s a physical response to stress and do your best to manage it.
Work off the adrenaline, take long walks, clean your garage. Stay busy.
Simple advice by @X8sEVSw-zcq , that we all know but fail to practice in these stressful situations. Nothing to do about unrealistic evaluations, the worst we can do is stress out.