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.
Witness to My Life: The Letters of Jean-Paul Sartre to Simone de Beauvoir, 1926-1939 (ed. Simon and Schuster, 2002) - ISBN: 9780743244053