Because my daughter just spent several weeks in India, some of them with the Missionaries of Charity in the homes for the dying, she and I have had some conversations about faith and bitterness and suffering.
Those who know me are aware that faith for me is difficult. I analyze to death. I don't trust emotions very much. The following series by Richard Beck has been meaningful to me as I struggle: The Varieties & Illusions of Religious Experience
If you only read a couple of the 16+ posts, you will be challenged to rethink or maybe revisit what doubt says about faith. A couple of the most meaningful posts for me are chapter 16--wish and ontology revisited and chapter 8--saints of darkness, where he writes about Mother Theresa's experience of "faith as the experience of 'not being loved' by God. Faith as owning 'darkness' and 'pain' of God."