ephemeral
Meaning: short-lived, fleeting
Usage: She promised she’d love me forever, but her “forever” was only ephemeral: she left me after one week.
evanescent
Meaning: fleeting, momentary
Usage: My joy at getting promoted was evanescent because I discovered that I would have to work much longer hours in a less friendly office.
transient
Meaning: passing through briefly; passing into and out of existence
Usage: Because virtually everyone in Palm Beach is a tourist, the population of the town is quite transient.