Imagine yourself in non-pandemic times making a great trip or even a whole Erasmus semester abroad. You are having a very good time and are maybe even a bit euphoric. Towards the end of your trip, there will be moments where you enjoy yourself a lot but also realize that this time will not last forever. That is euphancolia.
A word or phrase when put together has no real concrete meaning, but the meaning has become known through commonuse and realative familiarity with the speaker.