My wife had to learn it the hard way. She burned out and many problems psychological and physical started from there. The burn out doesn't come from one day on another. It takes a very long time and you barely notice the slow process. Tiredness... Sleepless... Depression... Aggression... All in small little doses, you never think of burn out. But one day it doesn't go further. She wasn't able to work anymore. I will not go into details, but it took over a year for her to recover and learn how to say no and how to reduce stress and its still not completely done till today. She's working again but have to be careful not to fall into old habits.