Just as St Paul's writing about suffering had "street cred" because he suffered so much for the sake the Lord, St Teresa knew whereof she wrote for the same reason, she suffered much, especially after her turn to the Lord, a reality most of us find disconcerting despite the fact the Lord tells us that to follow Him is to be led to the Cross.
At one point in her life, prior to her profound encounter with God, Teresa contracted malaria and became very ill. During her illness she had a seizure that left her unconscious and near death. In fact, she was thought to be dead. She awoke several days later only to discover a grave had been dug for her. This illness left her paralyzed for three years. But she was still not yet able or willing to give herself to God through prayer. Once she was given the grace of desiring to pray and the grace to turn to God in prayer, she wrote: "Prayer is an act of love, words are not needed. Even if sickness distracts from thoughts, all that is needed is the will to love."
What kept her from prayer, even for years after entering the cloister? It was a malady all too common, even in our day. She did not believe herself worthy of God's love and tender care. After she was able to truly pray, she asserted that not praying is akin to "a baby turning from its mother's breasts, what can be expected but death?"
Let nothing make you afraid.
All things are passing.
God alone never changes.
Patience gains all things.
If you have God you will want for nothing.
God alone suffices. — St Teresa
Sancta Teresa de Ávila, ora pro nobis