Formal:
* "La séparation d'une si douce tristesse" (literally "The separation of such sweet sadness")
* "Le départ d'une tristesse si douce" (literally "The departure of such sweet sadness")
* "Le moment de la séparation, si douce et si triste" (literally "The moment of separation, so sweet and so sad")
Informal:
* "C'est triste de se séparer, même si c'est beau" (It's sad to separate, even if it's beautiful)
* "C'est dur de dire au revoir, même si c'est bon" (It's hard to say goodbye, even if it's good)
* "On se sépare, mais c'est beau quand même" (We are parting, but it's beautiful nonetheless)
Literary:
* "Se séparer d'une tristesse si douce" (To separate from such sweet sadness)
* "Le départ, tel un doux chagrin" (The departure, like a sweet sorrow)
The best option would depend on the specific context and what you want to convey.