Praise God for salvation!!
That feeling is only the enemy who tries to made you feel insecure! For the Word says he came to steal, destroy and kill and when a person is insecure, about their faith, he knows he can put all things in your thoughts and draw you away from God. Stand firm in your faith for the Word of God is true and HE saved you therefore you can know you are saved.
Draw nearer to God everyday by studying the word and having a personal relationship with our Lord and you will see how your spiritual life will grow.