The speaker addresses an inner force she calls “Nucleus”—the core of herself, a creative seed—as the power that holds all her extremes: heat and cold, strictness and ecstasy. It decides whether she acts or sinks into stillness, and it fills her nights with intense, almost overwhelming dreams. She feels both subject to it and grateful for it; the repeated “my” makes the relationship intimate.
Calling it a “Little Seed” that “flowers to such splendid seeming” suggests that this hidden, small thing can turn into something beautiful when she needs it most. There is a quiet, reverent, almost worshipful mood, but it is also full of warmth. Instead of sounding oppressed by being controlled, the speaker sounds like she is writing a love letter to the source of her energy, imagination, and changing moods.