At the beginning of 1921 Klein leaves her in-laws in Rosenburg and moves to Berlin. Other psychoanalysts have also left Hungary due to the intensifying anti-Semitic climate, including Sándor Rádo, Alexander, Schott and Balint.
After a few weeks spent in a pension in Grunerwald, Klein moves to Cunostrasse, a drab and uninspiring area. She has Erich with her, now six years old. Melitta, aged 17, is finishing her studies in Budapest, and Hans, aged 14, is at boarding school.
Klein delivers another paper on early analysis at the 1922 International Congress. On the back of this and her paper of the previous year, she is made an Associate Member of the Berlin Society.
After being made a full member of the Berlin Psychoanalytic Society in February, Klein embarks upon her first child analysis. This marks the start of a bold new approach to analytic treatment and theory, and the start of Klein's career. This is only strengthened when Klein's paper, 'The Development of a Child,' is published by Ernest Jones in the International Journal of Psychoanalysis.
The child Klein names 'Rita' in her notes enters analysis with her; she is only two and a half years old. In November Abraham, at that time supervising Klein's work, writes to Freud:
"In the last few months Mrs Klein has skilfully conducted the psychoanalysis of a three-year-old with good therapeutic results. The child presented a true picture of the basic depression that I postulated in close combination with oral erotism. The case offers amazing insights into instinctual life." (A Psycho-Analytic Dialogue, The Letters of Sigmund Freud and Karl Abraham, 1906-27 [Hogarth Press, 1965], p. 339)
Meanwhile, in her personal life, Klein and her husband Arthur attempt reconciliation, moving into a large house built by Arthur on his return from Sweden, Auf dem Grat 19, Dahlem.
Freud publishes The Ego and the Id, his second and definitive structural theory of the mind. It marks a turning point in his theory.
Eager to learn from one of the great pioneers of psychoanalysis, Klein asks Abraham to analyse her. She manages to persuade him, despite his reservations about analysing a Berlin colleague. At the beginning of 1924 her treatment begins.
After several months of trying to repair their marriage, relations between Melanie and Arthur fail to improve. Melanie leaves her husband for good in April, shortly after her daughter Melitta's marriage to Walter Schmideberg, a Viennese doctor and family friend of the Freuds.
Following this final breakup of her marriage, Klein moves into a pension at Augbwigerstrasse 17, where she struggles to keep custody of Erich against Arthur's opposition.
Six months into Klein's new analysis, member of the Bloomsbury group, Alix Strachey arrives from England. She is to become a key catalyst in the development of Klein's career.
Klein begins several analyses of young children, notably those she refers to as 'Peter,' 'Ruth,' 'Trude,' and 'Erna' in her notes. An important paper based on these cases is presented to the Berlin Society on 12th December.
A letter from Alix Strachey to her husband, outlining Klein's 1924 Berlin Society paper, stimulates great interest when read to the British Society on 7th January 1925. Klein subsequently plans to give a series of lectures in London, with the enthusiastic encouragement of Ernest Jones. The Stracheys are greatly supportive of Klein's visit, translating papers, tutoring her in English, and preparing the ground for her ideas in the British Society.
During the spring Klein meets Chezkel Zvi Kloetzel, a married man and father of one, at a dance class. They begin what, at least for Klein, is a deeply significant love affair.
In July, Klein travels to London for her lecture series, which is hosted at the house of Karin and Adrian Stephen (younger brother of Virginia Woolf) in Gordon Square. She gives two lectures per week for three weeks, to a fascinated audience. During her time in London, Klein meets Susan Isaacs, thus beginning an important and enduring professional and personal relationship.
Alongside these exciting developments in her career, Klein also suffers a great loss. Abraham falls ill in May, deteriorating steadily until he finally dies on Christmas Day 1925. Klein has been in analysis with him for only a year and a half. She later describes the termination of her analysis and Abraham's death as 'very painful'.