The speaker admits to something small but telling: they have nearly forgotten how to smile. Almost immediately they brush it aside with "but i'm fine," and then keep insisting on it. They don't need to smile at "stupid things," they don't want to fake a laugh, and they say they've lived with grey and black for so long that it no longer bothers them.
The repetition is what gives the feelings away. Someone who is truly fine rarely needs to say so three or four times. The poem reads like a person trying to convince themselves, or to shut down a worry before anyone raises it. The colors grey and black suggest a long stretch of numbness or sadness, and "too long" hints at tiredness underneath the calm tone.
The last line, "i know its gonna be fine," tilts toward hope, though it sounds more like a quiet promise than a certainty. The mood is flat and guarded, with a small flicker of resilience at the end.