The speaker has spent a long time pretending to be unbreakable. The first stanza describes a belief that hiding behind a mask would keep them safe from pain and tears. The second stanza turns to the smile itself, repeated three times, which looks real but is really a "perfect facade." The repetition makes it feel stuck on their face, something they can't take off even when they want to.
Then everything changes. Someone's careless, hurtful words undo the speaker "in one moment," and old feelings come rushing back. The poem suggests this person has hurt them before, or that the words reopen an old wound. The speaker responds by writing, which seems to be their way of working through the hurt, and then by retreating to a corner to be alone.
The mood is quiet, sad, and a little defeated. The simple, repeated phrasing gives it the feel of a diary entry, as if the speaker is admitting the truth only to themselves.