Another reason for lack of assurance is that we tend to trust our feelings rather than the Word of God. We somehow expect some kind of continuous emotional bliss.  When this is missing the doubts begin to come. We have to be committed to Christ and His finished work at Calvary.  The Apostle Paul told the Philippian church, "And so I am sure that God, who began this good work in you, will carry it on until it is finished on the Day of Christ Jesus." Phil 1:6 The Good News Bible