I will try to explain using my perspective and the little knowledge that I acquired, but I am not native neither a professional teacher, just a student.

Separation is a NOUN, so I can say. My SEPARATION was like a cold war, without any shooting but with a huge pressure and a heavy atmosphere.

Separated, it can be an ADJECTIVE or the past tense of a VERB= I think she is a SEPARATED girl, so why I cannot try something with her, considering that I am SEPARATED too.