Everyone you've ever met is an experience. Some people stay long enough to see different versions of you; others enter your life for just one season. They come to teach you something or to show you what you've been settling for. And just because that season ends doesn't mean it was meaningless.
But here's something you need to hear: be careful not to use detachment as an excuse. Don't hide behind "everything is temporary" to avoid vulnerability, to avoid accountability, to avoid the hard conversations you don't want to have.
Letting go isn't the same as running away. Sometimes people leave because it was their time. Sometimes they leave because you never let them in. Know the difference.
What wasn't meant for you was never meant for you, and what's meant for you was always going to be yours. Trust that, but don't use it as a reason to stop trying.