One thing I realized after some rather ugly burn out a few years ago is that I mostly created my misery myself - there were demands and questions if I can take on some work but no psychopath managers, no deadlines that couldn't be moved.

I created most of the pressure myself - my biggest sin was working more hours than being paid because I wanted the project to succeed. It was never appreciated and only lead to be being angrily called on Sunday if something was broken - something my lazier 9 to 5 colleagues never experienced.

So I'd say instead of practicing no, first practice setting boundaries and practice organizing yourself.

Additionally a perverted mechanism came into play because I cared and wanted to improve the product at the time - the others kind of said: do if you like but that came without any real support and it lead to me being kind of laughed at as a hot air talker just because I was the only one that wanted to improve things.

Looking back I should have quit way earlier and of all things I shouldn't work overtime for free.

Having to say no could have been avoided if I had a good schedule that I could have used to prioritize the orders and giving the person making the demands an overview of my workload.

Watch out if there is some unhealthy inertia in your organization and especially be careful doing important silent work. It won't be thanked.

Either quit and look elsewhere or watch out that your workload is fair and similar to your colleagues even if that means to not solve that interesting problem because it's not your official job.