When you, my lover, on a bamboo horse, Came trotting in circles and throwing green plums (3-4)
These lovers go way, way back. They were practically in diapers when they first met. Well, at least the speaker's lover was still riding a toy bamboo horse, which suggests that he was pretty little. The point is, the speaker's evocation of meeting her sweetheart in childhood gives us a sense of how deep their relationship—and their love—is. It has its roots way back in their childhood.
…At fourteen I became your wife, So bashful that I dared not smile (7-8)
The lovers get married, but the speaker's so shy she can't even smile. We know that feeling. Don't we all get tied up in knots when we're around our sweetheart? The marriage between the speaker and her lover represents the fulfillment of their love for one another.
But at fifteen I straightened my brows and laughed, Learning that no dust could ever seal our love, (11-12)
Here we see the speaker growing more comfortable in her relationship with her hubby. She's finally laughing (thank goodness!). Her statement that "no dust could ever seal our love" is ambiguous. Dust evokes many things—it evokes burial (since we're buried with dust, or dirt) and it also evokes travel, a "dusty" road. So here the speaker could be suggesting that not even death or separation could destroy their love for one another.
That even unto death I would await you by my post And would never lose heart in the tower of silent watching (13-14)
In these lines, we get a stronger sense of the speaker's love for her hubby. She's willing to wait for him even "unto death," so she's really into this dude. By saying that she would never lose heart while "watching" for him, she again emphasizes how her love for him will never wane.
And, because of all this, my heart is breaking (25)
Oh no—the speaker's heart is breaking. Why? Because she misses her sweetheart so much. The image of the breaking heart here really gives us a sense of the toll that the husband's absence takes on the speaker. The more we love someone, the more our heart breaks when we're separated from them.