I hear from many people who are frustrated with all they have to get done during the day. Many times, they will come in with a plan to get things done, and their boss will give them other tasks to do. The common refrain is "I don't have time to do it all and I can't tell my boss no".
I can solve that problem for you - simply ask "what would you like me to not do?" and show your task list.
When you have too much to do, something won't get done. Asking this question puts the responsibility on the manager. You essentially are saying "you pick what won't get done."
The manager may assign the task to someone else (now that they understand your priorities) or they may pick a task to not get done. Either way, it is you saying "yes", but negotiating on what gets done.
The one thing you shouldn't do is take on the task and stress out. That doesn't help anyone.