I really recommend "The Art and Craft of Problem Solving" by Paul Zeitz, who coached the winning USA team in the International Math Olympiad: https://www.goodre

I really recommend "The Art and Craft of Problem Solving" by Paul Zeitz, who coached the winning USA team in the International Math Olympiad: https://www.goodreads.com/book/show/593458.The_Art_and_Craft_of_Problem_Solving https://www.goodreads.com/book/show/593458.The_Art_and_Craft... One of the things he focuses on is repeatedly challenging yourself with problems you don't know how to solve, and to stick with them and keep trying different angles and approaches. Even if you don't make progress, you'll be conditioning yourself to tolerate longer periods spent working the problem without a payoff.