In just three lines, the speaker describes herself as a contradiction: young in years, but built like an old temple. A temple suggests something that has stood a long time, that people treat with awe, and that has survived whatever has been thrown at it. The comparison makes her sound sturdy, sacred, and older in spirit than her age.
The second and third lines turn toward another person. Someone tried to "break me down" and failed, and that failure frightened them off. The poem flips the usual story. Instead of the speaker being the one who is abandoned or damaged, the other person is the one who cannot cope, because her strength shows up their weakness or their intentions.
The mood is calm, proud, and a little defiant. There is hurt underneath, since someone did try to break her, but the speaker doesn't dwell on it. She states her endurance plainly, and that quiet confidence is what gives the poem its force.