The speaker is trying to stay calm while something exciting happens, most likely the beginning of a romance, though the poem never says so outright. At first they feel surprised and delighted, but they quickly tell themselves to be careful. Every laugh, look and gesture becomes a "clue" to be read, and the situation is "sticky" and "confusing."
The middle of the poem shows the speaker's mind tiptoeing along, noticing every note and returning every smile. Short, soft words like "Gently," "Slightly" and "Unobtrusively" slow the pace and make the speaker seem almost to be holding their breath. Words like "Even, steady, careful / Balanced, poised" sound like a pep talk for staying in control.
Then the control breaks. "Ah! Heart, why do you totter!?" is the turning point, where feeling overrules caution. The closing image of being "in too deep" in the water leaves a mood of nervous excitement, half thrilled and half worried.