All people who take up the cross and follow Jesus in their lives, whether they are single people, married, or serving Jesus in the Church as priests or nuns, have ups and downs in life. This is the nature of human life and is to be expected.
Mother Teresa founded a new religious order, called the Missionaries of Charity. She experienced some problems with the local Archbishop in getting approval for the order.
Mother Teresa refused to have a bank account and did all her work with the grace of God, manifested in help from people around her. Shopkeepers would throw loaves of bread over the fence, and Mother Teresa would take it, go out, and feed the orphans in the slums.
God taught Mother Teresa, Keep the faith and drop the fear. Don't believe your doubts and doubt your beliefs. Life is a mystery to solve not a problem to resolve. Trust me. Life is wonderful if you know how to live.
Thus, Mother Teresa put all her faith in God, and served him in the eyes and in the hunger and in the needs of the poorest of the poor.