[sys_64738]

> My boss would ask me to dive into another tool I don't really know and the cycle would repeat ad nauseum.

There is power in saying No.

[unnouinceput]

There is power in saying "Yes, for this price" too. Explain the problem and the damage to your mental health if you do that, hence the price.

[afarrell]

Your first sentence is correct. Your second sentence would be better revised to "Explain the problem and the fact that the client will be signing on for a long timeline and high level of schedule-risk. Then work at a pace which does not damage your mental health."