Of a truth and rightly said, we are a product of our thought.
And that's why no thought must pass through the garden of our mind without us been conscious of it. Because like you rightly said,it will surely manifest either good or bad. And since we prefer good, then we need to dutifully guard our heart to permit only good thoughts.

Proverb 4:23 says: "Guard your heart with all deligent, for out of it are the issues of life"
It takes effort to guard the heart and may be a difficult task, but it is not an impossible one.
Thanks