Well this may or may not be helpful, and I should apologise for the rambling, but I have a desk job and I have had some coaching and general training on this. Things that helped me:
First ten minutes of working day, picking out one priority (only one) that I needed to do
Using a Pomofocus timer to break up my day and making those breaks mandatory (obviously not if I'm in an hour's meeting 😅)
Shutting email and Teams screens for periods of the day (this may be five minutes, maybe a whole Pomo or longer - depends on your job). Checking them after my five-minute break not before (otherwise it will play on your mind)
Trying to take at least a five-minute lunch break, even just to walk around the building a bit
Reassessing what I'm doing at the start of every afternoon - is it helpful/working/am I the person who should be doing this/should I delegate (big one for me)
At the end of the day, taking ten minutes to make a list of things to do on my next working day and emailing it to myself (scheduled to arrive at the start of my working hours).
Walking or cycling to and from the office, or taking the dog for a short walk at the end of the day if I'm home-based.
Headspace app
Leaving my phone in another room in the evening while I read or watch TV (otherwise perpertuating the feeling of always being on/waiting for a message)
Bedtime yoga with Adriene
Magnesium supplement