Busy people are used to getting an overview of what is coming to organize themselves better, delegate what they can, ignore or postpone the less critical items and focus on what will get the needle moving.
Unfortunately, there is simply no way you can organize and plan what you cannot see. To address this problem, the first thing you need to do is bring everything in a single workspace.
Connect all your third-party services such as email clients, chat services and project management tools to a single system.
Once centralized, it will be easier to decide what you should focus on. It will also reduce the chances of a forgotten item disturbing your schedule.
Note that unexpected matters will always come your way in a chaotic environment that will need to be taken care of urgently. This exercise aims not to plan your time to the second but rather to make sure that you have an overview of apparent gaps that can be addressed and have a process to find those gaps.
Once you have a centralized 360-degree view, you can budget, prioritize and plan better.