Write automated tests for your code as you're writing it. Hitting the entire system is fine. If your code is properly factored then it should be easy to do. If not, then you get to learn how to factor your code.
Test your backend with real data, not playsets. So use millions of records instead of 3-4.
If you're touching database, then learn SQL, and learn the tooling for your database. Learn how to read a query plan, use the optimiser and design indexes.
My pleasure. Hope that it helps. If you check out some of the "TDD with .Net guides" or "TDD with Java" (Pluralsight, Youtube) then you can see the flow in action.
Backenders struggle with the same thing in the frontend but in reverse. For frontend, you need to be interacting constantly - the shorter the loop between code and frontend, the better.
Test your backend with real data, not playsets. So use millions of records instead of 3-4.
If you're touching database, then learn SQL, and learn the tooling for your database. Learn how to read a query plan, use the optimiser and design indexes.