[bethly]

When TDDing the tests are going to change a lot, just like the code you are saying you write anyway. I recommend the book Refactoring for the tools so you can make the changes without breaking the test, then clean up the now-unimportant tests, and then clean up the unused code. That way the tests stay green even when making major, breaking changes.

Being able to slice projects that way, in terms of functional behavior, is absolutely a skill that requires practice. You are already thinking about what your code needs to do: the practice is to get good at expressing that in terms the computer understand. Once you are in the habit, the tests make it easier to evolve the code rather than harder.

It is possible to be a good developer without that skill, but learning and practicing the skill will make you better.