You ask two questions.
One: We would live differently if we knew we were going to die. Why, then, do we not live differently, given that we could die anytime?
Two: Why are we not in constant fear of death?
Constant fear of death would be too taxing – we therefore (subconsciously) prefer not to believe we will die (answer to Q2).
Once we do not believe we will die, there is no need to act differently (answer to Q1).