WordNet sense 1 (retained; not shed):

lasting

WordNet sense 2 (continually recurring to the mind):

haunting

WordNet sense 3 (impossible to forget):

unforgettable

WordNet sense 4 (never-ceasing):

relentless, unrelenting

WordNet sense 5 (seemingly without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series):

continual