Learning to do what you are told by your manager is an important skill at the start of your career and some people find it hard to learn.
I once had a new employee at the start of his career and he simply could not bring himself to do as he was told to do.
After his first 8 weeks or so I said “you’ve got a week to start doing what you’re asked or you’re fired”.
He started doing what he was asked and became a star employee.
Also if you don't already keep a job list, start. When people, especially your boss, look to add something, say you are full capacity and they have to decide what to remove to offset. It helps 1) keep requests in check 2) will force managers to prioritise 3) Creates agree trails what doesn't get delivered.
Unfortunatly admin like this further reduces your working hours but usually is the best way.
Your boss told you to say no. You tell people “no”, and refer them to your boss, and keep your boss informed of what your current workload is.
You are drowning in work because even after being directed to do so you refuse to stop accepting new requests when you are overloaded.
Saying “no” is an important skill.