How do I get better at hard thermal-fluid problems?

You handle the standard thermal-fluid problems now. The complex ones still trip you because they layer several effects at once, and the clean textbook path stops working when heat, flow, and change all tangle together. Getting stuck there is not a gap in your ability, it is the frontier where the real difficulty lives. Complex problems yield to being broken apart. When everything seems coupled, find the dominant effect first, the one term that matters most, and understand that alone before adding the rest back. Draw the system. Name your assumptions out loud. Check the units and the rough size of the answer before trusting the detail. Most of the time these problems are not beyond you, they are just too big to hold in one piece, and the skill is cutting them into pieces you can hold. Take one hard problem and, before solving anything, simplify it honestly, what can you ignore, what dominates, what is the simplest version that still resembles the real thing? Solve that, then add complexity back one layer at a time. That patient decomposition is the whole craft. It grows every time you refuse to panic and cut the problem down instead.