Yesterday I thought "when I lose you" implied death, rather than simply a breakup of a relationship. But this morning it doesn't look that way to me.
I still think "like a room" doesn't fit with the rest of the imagery, which is all outdoorsy. The sheltering linden tree, the garden with specific aromatic plants in it...there's definitely a vibe of interiority here, but I just can't reconcile those secret-garden images with the notion of a human-built "room."
I do concede that the poet/narrator claims that it is his own words, and nothing else, that are doing the closing, and sealing the beloved into the privacy of sleep, which he cannot join.
Her secret garden remains inaccessible to him, just as her eyes closed in sleep exclude him, and her sleeping but still-sexually-arousing body excludes him. But he's claiming that it is his words that are doing this, and when the relationship ends, she won't be able to sleep because...her next lover won't be a poet, and won't be able to resist waking her for sex?
Just thinking out loud. Maybe something here will be helpful. Or at least amusingly wrong.