as described in dictionaries, the suffix -ism
has multiple meanings to them, and two stood out:

an "abnormal state or condition resulting
from excess of a (specified) thing".

and

a "manner of action or behavior characteristic
of a (specified) person or thing".

similar to both meanings, nostalgiaism to me
is the act or state of being drowned in nostalgia,
regardless of the positive and negative associations from it.