What? You say you take on too much every day? It's common and often the reason for that is not wanting to say "no".
See if this helps.
1. If you find yourself blurting out "yes" under pressure, then practice not answering the question. Give yourself the chance to make a decision with less pressure - give yourself some space.
Dwight asks you to sit in on a meeting next Monday.
You reply: "Let me get back to you."
Use that time to think about it and check whether it fits your priorities, then get back to them. Sometimes, taking a breath allows you the strength to defer or explain why your schedule doesn’t allow time to do that thing well.
2. If you feel you can make good choices on the spot but need a reply that doesn’t mess with your schedule, instead, focus on timing.
Micheal asks: "Can you prepare a … ?"
You reply: "When is the latest I can get this back to you?"
Now, you have room to schedule the request in a way that fits your plan. Having control over when you do it may make the task more feasible.
3. If your schedule is packed, but the person asking signs your paycheck, use "if/then."
Jan asks you to work on a new task that is needed right away.
You reply: "If I do this, I'm not going to be able to get to X, Y, and Z. Is that okay with you?"
This reminds them of the other projects you are working on and gets them involved in prioritizing.
You can't change how much time there is in a day, but you can change how you use it.