It flows
differently for them:   

Of course,
being immortal affects how the nations experience time. They’ve lived for
centuries/ millennia and experience time at a much faster pace. A year, let
alone a decade, for a human is nothing to them.

Ex: Canada once
spent an entire weekend in a trance, staring into Kumajiro’s eyelashes. [x]

Ex: Because time flows differently for him, by the time America visits his child/ later teen friend Davie again, the latter’s had both children and grandchildren [x].

Interestingly,
the nation’s immortality can also rub off on other beings.

Ex: Pets like
Hanatamago, Pochi, or the Gilbird’s have extended lifespans as a result of
being around their owner nations. Should they separate, they would become
normal pets again. [x]

Ex: The
situation with humans is similar, but not quite the same. Unlike pets, humans
have a stronger developmental capacity to perceive time. It’s likely for this
reason that being around a nation for too long would cause them to go mad. The
drastic warping of time would be too much to handle.

How the Nations cope with their Immortality:

From what we’ve
seen in the strips, immortality is depicted to be more of a curse than it is a
blessing. There are several instances where nations have expressed regret over
their existence.

Ex: Germania
and Rome, as fallen ex-nations, experience relief over not having to deal with
wars and their leaders.

Ex: As an
Ancient in his own right, China romanticizes about the past, wishing that the
world was the same as it was 4000 years ago.

Ex: Italy
admits that he sometimes gets bored of seeing the same people every day [x]. Also
notice how his boss recognizes that immortality isn’t beneficial in practice,
as Italy’s had to endure countless painful memories and experiences. 

Ex: The nations
have gaps in their memories [x]. 

While it would
be impossible for them to remember everything, as they’re not perfect immortals
and can get injured and die, the fact that Russia claims that it’s good that
they don’t remember all the bad in their life may allude to an unconscious repression
of memories.

Still using
Russia as an example, given how he’s historically been tortured and punished by
his bosses, it’s easy to understand why he wouldn’t want to remember everything.