forever vs constantly

forever and constantly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
forever	No	No	No	Yes
constantly	No	No	No	Yes

As adverbs, forever and constantly are synonyms defined as:

Other synonyms of forever include always, incessantly, perpetually.

forever (adverb)	constantly (adverb)
for a limitless time	invariably
for a very long or seemingly endless time	without variation or change, in every case
invariably