Word/phrase to mean something that just happens once

In your example you could say, "if life is lived just once." Alternatively you could say "a one-time event."

I would say "one-off":

one-off

n

brit

a. something that is carried out or made only once

b. (as modifier) a one-off job Also one-shot*

The typical turn of phrase is one life to live.

If I only have one life to live, why can't I live my life in my own way…

For the life issue in the body of the question, previously-suggested one-time thing is what I would use. However, for the title question, "Word/phrase to mean something that just happens once", previous suggestions like unique, one-off, one-shot, and singular all apply in various situations, and the following apply in other cases of things that happen only once:

• singleton e.g. "A class that may not be instantiated more than once"
• nonce, "The one or single occasion"
• For words that "occur only once within a context" there's the versatile phrase hapax legomenon