Isn't this what probationary period is for? To know if you like this job and leave if not.
If they leave this job for a new one they'll still be burned out, just like the last two times in the past year they left their job for a new one.
My advice is the same as other commenters, OP should seek professional help with therapy. There are other things that can help like meditation, but in my experience there's no quick fix.
Sometimes it's not even about job. I have ALL the classical symptoms of burnout, but I still love both my jobs (full time and part time after hours) and would like to continue them, but I just can't gather enough focus to be as productive for the whole day. Fortunately, some light meds, workout and fresh air is starting to help.
You may struggle for time to exercise from the sound of it, but even a little bit helps.
It's important to be consistent because it takes some time to start noticing the benefits.
Also, create a bed routine and stick to it. For example, taking a hot shower, brushing teeth and then going to bed right after help to slow down a racing mind.
Reading a book before bed as well, I'm always impressed how much I can read and how slow the time goes by and I'm already sleepy.
"Reading a book before bed as well" - doesn't help everyone, I get too excited with story and have trouble with stopping reading when the time comes.