Yes, we know, intellectually, that everything is impermanent and that death is certain, though we are uncertain about when death will come. Yet we continue to deceive ourselves that death will not happen to us tomorrow or soon. We continue to grasp at all the things we desire and at relationships we are strongly attached to, believing(in our deluded minds)that they are there for us and forever.
However, there are moments of realizations, especially when we hear a powerful teaching on Death and Impermanence. We also panic that death can also happen to us anytime when a close friend or relative of ours falls sick and dies at a relatively young age, or dies suddenly in an accident. But these realizations only last for a while. Then the distractions of this life will pull us away and we will again be caught up in the affairs of this life and all thoughts and fears of death and Impermanence will fade from our memory.
I think the only way for us to make this realization remain firm and stable in our minds is what has been urged and advised by a great Master - do a daily meditation on Death. Like that, we are being made to focus on Death and Impermanence as a reality as opposed to the lack of essence and impermanence of all the objects of our daily mundane pursuits and grasping.