The more work you take on, the more time you spend managing your workflow rather than actually doing the work. Elizabeth Grace Saunders writes for Harvard Business Review about how to commit to less.
The Non-Committal Type
If you allow them, meetings, emails, and other forms of so-called status updates will ensure that your work is perpetually in a status of “in progress.” One tip to avoid this is to take a pause if possible when asked to take on more work. Use this moment to get clarity on the scope of the work, and ask “if it would be reasonable” for you to take some time to examine if you can fit the request naturally into your schedule. If you do decide to take on more work, notify stakeholders involved in your other projects and revise their expectations accordingly. Of course, if you already know you have no time for more work, give a polite, “That sounds like a great idea, but unfortunately, I am already fully committed right now.”
For more practical productivity advice, you can read the full article here: https://hbr.org/2015/01/accomplish-more-by-committing-to-less