Is Nirvana eternal?

In the book, _What the Buddha Taught_, Walpola Rahula writes on page 65, _"It is the vague feeling 'I AM' that creates the idea of self which has no corresponding reality, and to see this truth is to realize Nirvana, which is not very easy."_ I understand this to mean that 'Nirvana' is "seeing a particular truth". Therefore it seems reasonable to conclude that not seeing this truth is to not achieve Nirvana, and that losing sight of this truth is to lose Nirvana. So, is Nirvana an attitude or capacity or insight that can be lost, or is it a solid destination or eternal achievement as some claim/hope?