If you're thinking too long then you're doing too much

When Test Driving code with TDD try to get into a short cycle rhythm, 30 seconds write a test, 30 seconds to make it pass, x seconds to refactor. If you're taking much longer for the red/green steps then step back and have a wider look at the problem, maybe it needs breaking down further or perhaps you don't understand it quite as well as you thought you did. Go have a break and give your brain room to breathe, swap with your pair if you are pair programming or do something completely different for a while. Learn to recognise that sometimes struggling is an indication that you have to slow down, back up and reassess, not hunker down and plough on regardless.