Let's use an itch for an example.

Forever, is endless. You have forever to get used to it. There is no reason not to get numb to it. You adapt to it. You become content with the falling

Eternity, however, implies an end. But an indefinite length of time. Yeah sure you can get used to it, but what happens when it runs out? After eternity "expires"? You're left with an indent. A feeling of something missing. A feeling that a part of you is lost forever in the abyss...