Not to sound snarky, but sounds like you're doing it wrong. You're right that the canonical example of TDD is to write a test that fails and write code until the test passes, but that, IMO, is an over-simplification that ignores the motivation behind TDD. You should write tests whether or not you do TDD, but TDD ensures proper test coverage. If you write tests first, you ensure every line of code is covered by a test.

Otherwise you have to do what I had to last month, which was go back—after the fact—and write unit tests for a large chunk of our code base because we didn't have an automated way of verifying that our logic worked with different (read: non-happy path) data sets.