It i hard to advise when we can't sit with you and go through what you need to do.
One way I used to get rid of the stress is to plan the week like this:
Plan every monday morning in as a planning session, repeated every single monday, into your calendar.
Plan every friday afternoon in as a wrap up session, so that you know in advance what needs to be carried over to next week.
Monday morning, list everything down for that week, and anything that needs more than 15 mins, plan it in as a task into the diary. As in, an actual time span into your working calendar. If it doesn't need doing that week, put actual time aside for it when it does need doing.
Make sure your important stuff is all diarised. Monthly reports, give yourself time in the diary to do them. One to one sessions, book them in well in advance. Meetings, same.
Put aside wednesday afternoons for 'AOB' that comes in that needs to be scheduled.
External visits, training, etc, put in the diary as soon as you can. Delegate anything you can.
Anything that cannot be done in time, push back up to the boss as to which thing they want you to prioritise.
When you become a new manager it is overwhelming. But you can't do everything.
Write everything down that you remember as you remember it, have a notebook by the bed if you are waking up thinking about things.