The title does a lot of the work here. To be "pre-occupied" is to be busy, but it also suggests being occupied beforehand, with your mind already full before the world has a chance to enter. The speaker admits to being "busy with my own air," a odd, vivid phrase that sounds like living inside a bubble of private thoughts, breathing only your own breath. The result is a near-blindness: the speaker has been so wrapped up in themselves that they barely noticed what was around them.
Then the poem turns. After "all day standing," perhaps in place, stuck, or simply unmoving, the speaker walks in the garden at night. Judging by the shift, darkness seems to offer what daylight didn't: a chance to move, to pay attention, maybe to see differently. The mood is quiet and a little rueful, but also gently hopeful. There's no scolding, just a small, calm admission and a step outside.