Mother Teresa is a "saint" in the eyes of many throughout the world, but more importantly to those that she tirelessly served day to day in India.
Her goal was to bring peace to those that needed it the most, and any comments to the contrary should be viewed as suspect. During her life, she indeed experienced "doubt" and "darkness."
Many holy people throughout history have also experienced the extreme poverty of their souls, willingly presenting their lives in sacrifice for the salvation of others.
Her doubt, was not a "proof" for or against the existence of God, rather her life as an example is "proof" of a Mystery that reaches beyond our meager attempts at understanding.
She in fact did "let go" through a life of love and compassion. As she left this world and entered the presense of the Divine, I am certain she was welcomed by many...including Gautama Buddha.