Test Coverage all the more important

I start my testing career with the designation "Test Designer". My responsibility is to prepare test cases document for the one on going project.,
I had some sample test cases documents for the same project which prepared in earlier stage. But that time the project was in live. I don understand why test case documents were needed for this project that almost got finished.?
I started to go through the existing documents and prepared my own for every module in in the project, there was no clear documentation about the functional flow the project.
So i did Exploratory Testing ( That time i don know what Exploratory Testing means) and learn the project and prepare test case documents.,
In middle of the task only, i understood main objective of preparing this test case documents is to meet and ensure the Test Coverage of the project. On that time only i realize i was in the very important task and this documents go to sounds a lot in every project version releases.,
Before understand above things, that task was so easy to me, but after this, i felt like i was stand on the knife, If i missed out any test cases means, that may cause failure in production phase.
Oh, My goodness, i share my test case documents to development team and my senior testers and got valuable feed back from them, continually update the document.
This process get continues, every version release of the project, i updated the test cases documents of every module in the project. i didn't think test coverage is impossible that time, But it was one of my dream while i worked on the project, because as a tester we cant able to test everything in testing environment. It takes time to me to expand my mind to think out of the box.
The following alarm sound ring me in my heart when ever finish the test " I finish the test, but not complete test". I just want to stop this alarm, because some times, this sound broken my Peace, but at the same time, it give me aware about to learn new things to grow to attain my dream. I hope one day this alarm will stop :)