The speaker is exhausted and trying to hold herself together. She tells herself to keep breathing, to stay awake, to stay "lovely, pretty," as if she has to keep up an appearance even while she feels ill. The reason soon becomes clear: she has locked away her memories of someone she once loved, and touching that part of her mind feels dangerous, as if she might start burning.
The middle of the poem makes this pain concrete. She describes a hot summer day, a person climbing out of the water and sitting on sun-baked ground, then jumping up because it scorches their back. Remembering the relationship works the same way: something that once belonged to a warm, happy day now hurts to touch.
So she runs and hides the memories. The last lines turn almost childlike, blaming the memories themselves for being cruel. The mood is tense, weary, and raw, with grief that she cannot yet face.