It's also a little more natural to say: "I used to go there when I was in High School." - I don't know why, but that's what most natives would say. i hope that's helpful. :)

Today I went to *the* library after school… so *it* was very nostalgic. The word nostalgic isn’t really used to describe oneself, but rather something else. You can feel nostalgic or something can be nostalgic. Your sentence isn’t technically wrong, but to me it just sounds more natural to say “it was very nostalgic”