Four short lines, and each one begins the same way: "I forgot." The speaker isn't describing a sudden loss or a dramatic event. It sounds more like a slow fading, where happiness has slipped so far out of reach that even the memory of it is blurry.
The lines move from the general to the personal. First the speaker forgets happiness, then joy, then the look of a real smile, and finally the last and biggest thing: "how to be me." That last line changes everything. It suggests the missing happiness has taken the speaker's sense of self with it, so they no longer know who they are without it.
The repetition makes the poem feel flat and tired, like someone listing what's gone because they have no energy left to explain. The word "true" before "smiles" hints that the speaker may still see smiles around them, but ones that feel fake or empty. The mood is quiet, numb, and lonely, and the lack of any hopeful ending leaves the reader sitting in that emptiness.