How can I grow my faith in God? I feel I'm a bad person because I know that Jesus died for us but I still have sinned. I know that God forgave me, but when I look back I think that I have been really bad and I feel very sorry for it ... what is this feeling? Please explain.
You are not growing because you are spending too much time looking backwards. "But Jesus said to him, "No one, having put his hand to the plow, and looking back, is fit for the kingdom of God"" (Luke 9:62). Try plowing a field while constantly looking over your shoulder. It just won't work. We have to keep our eyes on where we are going. We must focus on the goal. "Therefore we also, since we are surrounded by so great a cloud of witnesses, let us lay aside every weight, and the sin which so easily ensnares us, and let us run with endurance the race that is set before us, looking unto Jesus, the author and finisher of our faith, who for the joy that was set before Him endured the cross, despising the shame, and has sat down at the right hand of the throne of God. For consider Him who endured such hostility from sinners against Himself, lest you become weary and discouraged in your souls" (Hebrews 12:1-3).
Paul spend a portion of his life persecuting Christians, but he didn't let his past affect his future. "And I thank Christ Jesus our Lord who has enabled me, because He counted me faithful, putting me into the ministry, although I was formerly a blasphemer, a persecutor, and an insolent man; but I obtained mercy because I did it ignorantly in unbelief. And the grace of our Lord was exceedingly abundant, with faith and love which are in Christ Jesus. This is a faithful saying and worthy of all acceptance, that Christ Jesus came into the world to save sinners, of whom I am chief" (I Timothy 1:12-15). Paul was able to do this by leaving his past behind him. "Brethren, I do not count myself to have apprehended; but one thing I do, forgetting those things which are behind and reaching forward to those things which are ahead, I press toward the goal for the prize of the upward call of God in Christ Jesus" (Philippians 3:13-14).