Set an alarm ten minutes before a block ends to find a stopping point, and another at two minutes to actually stop. Before you switch, write one line about what you were doing and the next step — it quiets the unfinished task and makes coming back much easier.
Then bridge the gap with the same short ritual every time: stand up, get water, breathe, read the first line of the next task. And remove switches where you can by batching similar tasks and clustering meetings, so there are simply fewer transitions to manage.