It answers the question that was tormenting you: my love, you are not 'one thing in my life' - not even the most important - because my life no longer belongs to me because...you are always me.
Jean-Paul Sartre, Simone de Beauvoir, Lee Fahnestock, Norman Macafee (2002). “Witness to My Life: The Letters of Jean-Paul Sartre to Simone de Beauvoir, 1926-1939”, p.275, Simon and Schuster