While this is the only time we find the word ‘advocate’ in the New Testament, the greek term behind it is found on another four occasions. On each of the other four occasions it is with reference to the Holy Spirit and is translated ‘Comforter.’
The essential meaning of the word indicates, “one called alongside to help” which is part of the ministry of Holy Spirit to God’s people, He has been sent by Christ to help you.
But in 1 John 2:1 it is with reference to Jesus Christ, and in this context John says that Christ is representing us in Heaven before the Father. On our behalf He pleads the merits of His own sacrifice upon the cross in order to obtain a sure forgiveness to those who depend on Him alone for acceptance before the Father.
So if you are crushed by the guilt of your sin, what you need more than anything is this heavenly advocate, Jesus Christ.