Is there a word for when you seek a nostalgic feeling of happiness but it won't come again? – sort of antonym of déjà vu where an old sensation comes of its own accord but you can't tell where from. Because I watch an old movie I used to love so much, only to find that the feeling is gone, and I want to tell friends about it but use a fancy word.
Or did I misunderstand déjà vu? What is it then?
I do think that your understanding of deja vu is a little off. Deja vu is a strange feeling that something has happened before, is oddly familiar, but could not have happened before. To me the opposite would be something that you know has happened before, yet seems completely new and unfamiliar.
It sounds like you want a kind of antonym for one aspect of nostalgia. I don't have a suggestion off the top of my head.
Here's some interesting stuff from Listverse, always fun to read: