Yes, Gulliver, I read the recent news about her - I am looking forward to reading the entire book. I love biographies of fellow strugglers.
It is at first glimpse surprising, but NOT surprising once we realize that she was a human being, after all, and burdened by all the human frailties that all human beings have. (This is the danger of sanctifying people, we impinge upon them all our dreams and unrealistic hopes.) Since, as I understand it, most of her life she experienced this emptiness, it sounds like either she lived an extended period of "the dark night of the soul", that mystics refer to, and experience, or, possibly, she never did achieve a sense of union with God, but was striving to do so because her religious tradition made that imperative upon her. Or she may have drained her energies with all her good works, which left little to work with when turning to the inner work, perhaps? Certainly she gave away her entire life to others.
We often confuse saints with humanitarians, or delegate the word saint to a humanitarian, who may or may not be a mystic, and it is entirely possible that Teresa was the latter, and obviously, was no mystic, at least that is the impression these letters give. Politically speaking, it was to the church's advantage to encourage that aura of saintliness, based on her extraordinary humanitarian efforts, but that says nothing about the depth of her faith or union with God, that is quite another thing.
On the other hand, she may have been an individual who was a true boddhisatva, one who returns to help those less fortunate, and thus has no interest whatsoever in her own redemption or salvation, because, if a true boddhisatva, she is already redeemed and saved. And her difficulty in achieving union or a sense of God was, in part, possibly due to that fact - there was no "other" in her soul, and thus she could fine no "other" out there to unite with. If this is true, then, the church, in its ignorance, insisted she conform to the lesser lights in the church, and achieve a "sense of God's presence", and because the church does not understand or grasp, or even allow the concept that God may be truly within, and NOT without, she had no method of advancing beyond an external God. And unfortunately, therefore, felt bereft and somehow a failure because she had no explanation for what may have been a truly beatific consciousness.
From this same perspective, which might be relevant: There is a failing I believe in the christian discipline, and that is that it does not address the ego, and the inevitable loss of ego strength that occurs with the advance of union with God. This loss of ego can be experienced as a loss of communion or love when it occurs, and unless understood and addressed, can more than likely create a permanent emptiness that appears to the individual to be lack of faith, whereas it is truly loss of the separated self. Perhaps it was this explanation that accounts for her struggle? There is no doubt in my mind that the woman was an extraordinary woman, who sacrificed her life and, evidently, in some way, her soul, in service to the sick and poor. For that she stands shoulders above the majority of human beings. Whether that made her more than human, and thus a true Saint, by the Catholic interpretation of saint, is another issue, I believe, than who she truly was and where she stands in religious history. Certainly she was greatly rewarded by the love and adoration she received from those she helped throughout her life - maybe that is all that matters in the end, from the perspective of the world. Only God and she truly know who she was from God's perspective.