You will die. That much is certain. It is only a question of when. And between your death and now, you will suffer much loss as well. It may be the end of a relationship or the death of a pet. It may be the loss of a job or something internal and more abstract like the loss of innocence. But it will happen to you just like it happens to everyone. And there is nothing you can do to stop it.
In Buddhism, loss is understood as a part of existence. Impermanence, one of the three marks of Buddhist teaching, is simple enough to understand: everything changes. This presents a problem psychologically for human beings. Human beings crave permanence and security but life is in constant flux. How are we expected to find peace when everything feels precarious and unstable? There are no easy answers here. If there were, people would generally be a lot happier and less anxious. In my experience as a psychotherapist, however, people are a lot more unhappy.