The speaker is pleading with someone who is slipping away. Nothing in the poem says who that person is, and that openness helps. It could be a lover who has pulled back, a friend, or someone who has died. Judging by the phrase "what I lost," the loss has already happened, yet the speaker still talks to the person as if they could be asked to stay.
What stands out is the repetition. Almost every line begins with "to stop," and the requests shift as the poem goes on: stop moving on, stop drifting, stop disappearing. Each one tries a slightly different way of saying the same thing, as if the speaker keeps searching for words that might finally work. The final line, "to stop leaving me," drops the vague language and gets personal. The mood is raw, tired, and desperate. There is no anger here, only a quiet, painful wish for the leaving to end.