It's encouraging to think that the Spirit of God often uses us without our knowing anything about it. This story that someone told me once is my favorite example:
A woman went on her first retreat. As she arrived at the retreat house she was very nervous; she wanted to "do it right" and "work hard" on her retreat. Soon she went to the first silent meal with the rest of the anonymous retreatants.
She liked the supper and wanted to help herself to a little extra, but worried that to do so was unspiritual. She saw a man there who ate a little bit and then quickly got up and left. She thought, "I should be like that guy; he's so spiritual that he eats only a little and then gets up and leaves, presumably to get back to prayer."
So she accused herself of gluttony and sensuality and of even thinking of ruining her retreat when it had hardly started. She left the dining room and began to walk to the chapel. On her way she looked outside and saw the man who had left the dining room before her--he was outside, smoking a cigarette.
Seeing this, she was able to laugh at herself: "Here I was accusing myself of sin because I thought this guy was so spiritual, and there he is, smoking." He hadn't left the dining room because of any holy motive; he had left for a smoke. At this she was able to get over herself, let go of the idea that the retreat depended on her, and begin to relax in the Lord.
For me, I love the story because the man never had any idea what the Spirit had done through him. He probably saved this woman's first retreat experience. Thus we might always remember that in spite of all of our tortured reflections and doubts, God is probably making use of us in ways we will never know on earth.