General Winter probably can't forget, but he also probably doesn't need to forget, either. I don't think he's really able to be hurt. Because he's Russia's winter. If anything hurts him, it's a character replacement. The Real Russia's Winter is probably nigh inconsiderable.

Hatred is made up by humans, but also really isn't something made up on purpose. It is one of those visceral things that is probably innate for humans, and is a part of reality. That said, Hatred could swing either way: if you interpret it to be a part of reality, or part of perception.

Actually, that's probably a good guideline.

Things that simply are, that are part of reality itself: external concepts, and probably can't be treated as entirely human. They kind of become deity-like in the manner of Death. You may be able to make 'sleep' generally sleepy, but you can't characterize it. It's beyond human ability to fully make them human. Hence the generations upon generations of trying to describe the 'ocean' as a woman... but making her more like a god.

Things that are perceived, that are interpreted or invented by an organism are impermanent like a human and probably can be treated as such. This is what gives stuff like Hetalia life-- because people can characterize these things, they can be made humanish.

If somebody in fanfic treats 'Spring' or 'Cold' like a real human being rather than a representation like Death, that instance is probably a character replacement and a Mary Sue only calling themselves that... You can't 'be' Death. Death is Death.

This is why the antics concerning Death from Discworld are so /funny/. They put an eternal, external concept that is part of reality and try and make it fit in with things that are humanish, but to no avail. Even his daughter ends up a little... off.

I don't think General Winter can forget. As the personification of a thing, he most likely has memories and powers beyond Nations. Since he is made of temperature, wind, water, geography (to my understanding of the concept of winter), he shouldn't be able to be neuralyzed.

Nations, however, are entirely the institutes of men. Only humans have created nations, only humans are parts of nations, thus the nations themselves are fully human in concept, appearance, and likely biology.

These are my thoughts on that subject.

As for which concepts on your list are external, Spring, Cold, and Ocean probably are. Hatred and Sleep are human and animal conditions, so thus might not qualify as external concepts. I'm no expert, so what do other people think?