It's difficult to know whether or not the term "fate" accurately describes the philosophy at the center of Tess of the d'Urbervilles and other works by Hardy. Fate implies a force beyond human control that directs our actions. I would tend to see it as not so much fate as a kind of randomness that causes the decisive (and tragic) turns of the plot in Tess. Added to this is the deliberate and very human folly present in the story.
Whether intentionally or not, Hardy is making a statement about the vulnerability of women in the nineteenth-century world. This is not fate, but the gender dynamic that has been dominant throughout history. Angel Clare cannot accept Tess's having had a previous relationship because this is the way most men thought at the time. The randomness of human events can be considered responsible for his not seeing the letter she has written to him. But even if he had seen it, the result would have been the same—he would have rejected Tess.
The other crucial point is that Tess never tells Angel that her relationship with Alec d'Urberville was non-consensual. The reticence on this point of not only Tess but of Hardy himself as narrator is typical of the time. But the whole story is also a chain of chance happenings beginning with the death of Tess's horse and concluding with Tess's returning to Alec and, finally, killing him. The overall point, however, seems to be that tragedy is caused by both human folly (and its corollary of cruelty) and the randomness of the cosmos.