You can work on one thing at a time. The more knowledge-intensive the task, the higher the switching costs.
Plan to clean your living room. Get out the dustcloth and the vacuum and the polish and the glass cleaner, then discover that everybody is in there, watching the big game. You have to do something else for awhile.
That isn’t as tough because you can just leave all the cleaning supplies out, and when you come back, they’re there and ready for you to start the task.
Your brain doesn’t work like that. As soon as you decide to do the grocery shopping instead of cleaning, your brain begins mentally putting away the dustcloth and vacuum and so forth, making mental space for remembering the milk and the vegetables. By the time you start back on the cleaning task, you’ve lost all the time you spent preparing.
If a task takes you an hour to complete, but half an hour of that is preparation time, and you keep switching tasks within the first 30 minutes, you will never complete that task.
Find anybody who seems to have a whole lot of stuff going on. I guarantee that you will find somebody with a lot of projects that are half-finished.