How is Tess's life a constant movement between hope and despair in Hardy's Tess of the d'Ubervilles?
The text does not confirm the idea that Tess's life was one of "constant movement between hope and despair." The definition of "constant" is something that is not changing, not variable; something that is uniform and regular (American Heritage Dictionary). In the story, there are three short seasons of happiness during which Tess has hope while there are vast expanses of time during which she has despair. This incongruity of number and duration of times of hope versus times of despair can't really be characterized as "constant movement" between the two.
Having said that and setting it aside for now, we can discuss how Tess's life moves between an inequitable distribution of seasons (or times) of hope and despair. We'll note though that it is this inequitable distribution of despair amidst few moments of hope that helps create the tragedy of Tess's life and suffering, and so the inequitability of these two fluctuations of life--hope and despair--is a critically important element of Hardy's message about society and its skewed definition of a pure woman, a definition Hardy debunks, rejects and redefines through a psychological study of Tess's life.
The three times in Tess's life, starting from when she was sixteen, when hope might be possible are when she was still at home planning and hoping to be a school teacher; when she was settled into working as the poultry keeper at Stokes-d'Urberville's manor near Trantridge; when she and Angel Clare were courting (though against her will and better judgement) at Talbothay's dairy.
At the manor, she would walk out on Saturdays at the end of the workday, with the other workers, to Chaseborough where they enjoyed themselves and did shopping. During this time she was content though not exactly hopeful, because her hope of being a teacher had been sacrificed to the family need to have a replacement for their deceased horse Prince and an introduction to the monied (though artificial) branch of the d'Urberville family.
While at Talbothay's, she was happy doing a good job at somewhat dignified dairy work and associating with the other country dairymaids. She was even happy in becoming acquainted with Angel Clare because he was of a higher social class and education thus appealed to her thirst for learning and refined ideas, and he appealed to her heart strings. While she may be said to be happy, though reserved still, it is hard to say she was hopeful because she refused every offer of marriage Angel made to her and made efforts to keep him at his distance and herself away from his notice (he had other plans and won). You might say the one time Tess was truly hopeful here was after Angel's confession on their wedding night of previous dissipated wrongdoing. She was hopeful for those few brief moments that he would forgive her past since their moral falls had been just the same ("'tis just the same!"), though Clare's was by foolish choice while Tess's was from seduction and force.
The despair that fills her life between these three brief instances of content, happiness and sporadic hope were a sacrifice of hope; seduction; a pregnancy; a child; a child's death; exile to a strange part of the country; turmoil caused by love that she knew could not succeed; rejection by her husband on their wedding day; penury and exile to a harsh unyielding land; unwanted reunion with her seducer who has come as her "master"; the return of Angel too late; her murder of Alec; her flight (with a few days of peace in Angel's arms); her imprisonment; her execution.
The most obvious state of exchange between hope and despair is found, of course in Tess' circumstances. Fluctuations between desire to grab hold of the potential treasures of life and the desire to let go of what fell into her hand flood the novel.
Looking beyond the obvious, we can also find the characters in a position of struggle with destiny. We see an ebb and flow between the natural state of created beings, in which there is supposed to be harmony, and the artificial state of modernity, in which rest for the soul is pushed aside for the progressive greed of man's own devising. It is in these settings that Hardy's ideas of purity and corruption are personified.
In the big picture, we see that while hope and despair are on opposite ends of the stick, Tess' life never actually veers to the right nor to the left of what has been purposed for her. There is no redemption - even in death. This pulls hard at the idea that there is no free will, signifying a battle in both Tess and Hardy against a God who both gives free will and redeems - a God Hardy was reluctant to accept. The Biblical themes blended with pagan festivals and rites of sacrifice are an incongruent mix that plays out as confusion, sorrow and death in the life of Tess of the D'Urbervilles.