Once you understand how to tackle a task, you will be able to complete it successfully.
Figure out which type of work yours belongs to.
Each time you get a task, assign it to one of these categories. This will help you understand what action you need to take to complete that task.
When you engage in doing predefined work, you are working on tasks from your calendar, i.e., you are completing tasks that you have previously determined you need to complete.
Work that shows up.
When you engage in doing work that shows up, you are working on tasks as they come up. These are tasks that you have not previously planned for.
When you engage in defining your work, you are simply categorizing your work into the tasks you will finish immediately and the tasks you will add to your calendar to do later (this becomes predefined work).