Estimation is a much more important time-management tool for a teacher than prioritisation. If a task can be done quickly (this means less than a minute) you should do it right there and then; if not, it will get dealt with by the end of the day.
This notion - everything gets dealt with by the end of the day - is at the core of reducing stress through time-management. You should sleep each night with a clear desk and a clear mind. Of course 'dealing with something' does not and can not mean that every task is completed magically each day; in many cases it simply means that it is added to your plan. However, if you are rigorous in following your plan that doesn't matter. If it is on your plan, it will get done. Reminding yourself and convincing yourself of this fact is at the core of using time-management to reduce the likelihood of stress. If you follow your plan rigorously, and mentally clear your to-do list every day, your life is pretty much under control.
You must remember that estimation is focused on achieving this specific goal: to ensure that our desks are clear at the end of the day. This doesn't mean shovelling an in-tray into a pending tray - it means completing a task if it can be completed, and - if not - adding it to our plans for the future.
What you must do is become a Guru of the Glance
There is nothing magical about practical estimation, and it often does not require anything more complicated than basic arithmetic and an educated eye. A guru-of-the-glace will routinely look for numbers in any task. A PowerPoint presentation? Glance in the bottom left hand corner to see how may slides are involved. Assume two minutes per slide, and do the maths. Minutes of a governor's meeting? Glance at the back page and note the page number; assume each page will take one-minute to skip-read, and do the maths. Flick through the pages, and count the action points; assume each will take - on average - five minutes to consider and implement. Do the maths. Of course, an introductory speed reading course would help in all this, but you can learn the basics very quickly. You should also familarise yourself with important numbers. If you have 50-minute lessons in your school, and you regularly mark 30-student classes, then you clearly need to mark six scripts every ten minutes. Cross stacking scripts allows you to monitor progress easily.