Do you know what causes doubt and lack of assurance? Deuteronomy 26-28

Deuteronomy 28:66-67 (NASBStr)

So your life shall hang in doubt before you; and you will be in dread night and day, and shall have no assurance of your life. In the morning you shall say, ‘Would that it were evening! ’ And at evening you shall say, ‘Would that it were morning! ’ because of the dread of your heart which you dread, and for the sight of your eyes which you will see.

In chapters 27 and 28 of Deuteronomy God is telling the nation of Israel that when they go into the land they can either live for Him or forget Him.   But, the walk of faith (for Him) or the walk of the flesh (for them) will come with some utter consequences.   These chapters are knows as the "blessing and the curses."   If you obey God through faith and walk in His ways, you will be blessed.  If you disobey God and in the flesh walk in your own ways, you will be cursed.   God spends more time in these two chapters outlining the curses.  He concludes with the above statement.   The worst curse you might have is no hope.   Instead of hope and a great view of the future, the only thing that remains for the disobedient person walking in the flesh is doubt and loss of any assurance in their lives.  When we decide to walk in the flesh we lose out on any assurances of God's love and God's care.  The blessings are, in fact, God with us or God without us.   In disobedience we will wish it was something other than it is.  At day time we will wish it is night.  At night we wish for the morning.   Lake of faith and trust in self will always cost us our assurance and bring in doubt.  The only way we can have a feeling of assurance in our lives is to have faith in God and walk according to His ways via faith.  The nation would not do that.   They would lose their assurance and have nothing but doubt.