The speaker starts to say something big: you are the air that I breathe. It's a familiar love-poem line, but the words stumble. "The air the" collapses, and "breath-" is cut off, as if the speaker is literally running out of air while trying to say it. Then comes the sharp "Wait!" The speaker stops, realizing the comparison doesn't hold up, because air is something you need to live, and they were alive before this person arrived.
That realization doesn't calm things down. It turns into two worrying questions: what did I breathe before you, and what will I breathe without you? The speaker can't picture life before the relationship, and fears life after it. The title, "Suffocating," fits this double meaning. The love feels so consuming that it takes the speaker's breath away, and the possible loss feels like being cut off from air entirely. The mood is anxious, shaky, and raw, with a sense of panic under the romance.