Nostalgia has a bit more bitterness than sentimentality. Sentimentality relishes the pure recollection of good times, whereas nostalgia is tainted by a belief that everything would be better if the person could just go back to those good times.
Nostalgia is the act of recollection; sentimentality is the character trait of one who often engages in nostalgia.
Nostalgia is an unavoidable outside force; sentimentality is a choice.
[Oh no, I’m not that smart…]
They are the same; they just have etymological roots in different languages.
Sentimentality is nostalgia applied to the present.
Alas, back in college I knew the difference!