Esther has been skiing, or is remembering the body’s insistence, and the heart’s brag is simply that it is beating. The novel came out in January 1963 under the name Victoria Lucas. Plath died the next month. The sentence is not a farewell note and not a piece of advice. It is a record of the body announcing itself against a depression that had argued otherwise. Read it as fiction that knew what it was talking about, and do not make her death the only margin.