Ella's poem is only three lines long, but it is built around silence. The speaker looks back at something, or perhaps many things, that never made it out into the open. The word "they" is never explained. We aren't told what these things were, and that vagueness is the point. The reader is left holding the same unanswered question the speaker has carried.
The feeling here is quiet regret mixed with fear. The first line says the words were impossible to speak, as if the lips simply wouldn't cooperate. The second goes a step further: these were stories the speaker was "always afraid to tell," which suggests a fear that lasted a long time, not a single moment. The last line, "the unspoken," is short and final. It turns all those hidden things into one name.
The mood is hushed, wistful, and a little sad. The poem also does what it describes: by not saying what the secrets were, it keeps them unspoken even as it talks about them.