What resources (books, videos, etc) to help develop math thinking skills?

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I am a applied mathematics major and have taken basic undergraduate coursework up to multi-variable calculus, ordinary differential equations, and elementary linear algebra. I am currently taking mathematical modeling and boundary value problems courses. I didn't struggle as much before with previous work but much of what I am doing now involves constructing and conceptualizing problems using assumptions and coming up with solutions without using a specific method. What is out there to help develop a better mathematical intuition and give approaches to solving modeling type problems?