It's odd that functional testing feels easier to you than unit testing; I find the opposite, and that unit tests force better structure into my code. I write fewer monolithic mega functions and more, smaller, simpler functions that can be unit tested.

Maybe it would help to approach some code in reverse to how you'd normally write it. Test-first development. You write your functional function, but make it full of stubs you think you'll need to call - top-down design. Then, write unit tests for those stubs - they can be simple at first, basically pass if they get some expected result, and fail otherwise. Then run your tests and confirm they all fail. Then, one by one, fill in the stubs, every time running your tests and watching as more and more tests pass.

This approach has a number of benefits:

• test-first development encourages writing smaller, more easily understood functions
• test-first development gives you high code coverage, which helps prevent bugs from creeping in during development
• there's a definite positive feedback endorphin reward from writing watching more tests pass as you develop, which encourages you to keep tests up to date and running clean

Test-first is easier in some languages than others, as unit tests are easier in this languages.

I hope this helps!