Can God use someone that has failed?
The Patriarch Abraham failed.  Genesis 12:11-20
God used Abraham to father a nation.  He is called the Friend of God, and Man of  Faith.   James 2:21-23.

Moses failed.  Exodus 2:11,12.
God used Moses to lead HIS children out of Egypt.  Exodus 3:10.

King David failed.  2 Samuel 12:9,10.
God used David to write many of the Psalms.

The Woman at the Well had failed.  John 4:17,18.
God used this Samaritan Woman to evangelize many people in her own town.  John 4:28-30.

The Apostle Peter failed.  Matthew 26:75
God used Peter on the Day of  Pentecost to preach a sermon and about 3,000 souls were saved.

God can surely use us.  Philippians 4:13.  I can do all thing through Christ who strengthens me.