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.