Sentence:  “Oh!” said Mrs. Venables, “how tiresome it all is. I'm sure you'll wear your brains right out with all these problems. You mustn't <WSD>overdo</WSD> yourself. [...]” 
Meaning of the word marked with <WSD> tags:  To give (someone or something) too much work; to require too much effort or strength of (someone); to use up too much of (something).