Sin is serious business. There are no little sins because, as these verses tell you, sin is unrighteous and fills you with unrighteousness. An unrighteous person cannot have fellowship with a holy God.
To further understand the seriousness of sin consider that your sins, the big ones and the "small" ones, caused Jesus to suffer a horrible death on the cross. The Holy, Righteous Son of God became unrighteous when He took your sins upon Himself. It took a BIG SAVIOR to atone for our sins.
So, how should you respond to such an amazing act of mercy and grace? You confess your sins to God. The word for "confess" means "to agree" with God about your sinfulness. When you "agree" with God about your sinfulness you also "agree" with Him about His holiness. As you come into agreement with God and enter into fellowship with Him it will begin to break your heart for how your sins made Him suffer.
That sounds a lot like Matthew 5:4, "Blessed are they who mourn....." Mourning over your sins and confessing your sins are essentially the same idea. And what happens when you confess your sins in a spirit of mourning, He forgives your sin and cleanses you from all of your unrighteousness! He is faithful and He will do it!
Being forgiven from your sin and cleansed from your unrighteousness sounds a lot like the remainder of the Beatitude, ".....they shall be comforted". What is more comforting than knowing you have been forgiven and purified by the grace of God in Christ Jesus?
If you want a better understanding of the second Beatitude, get your heart and mind around I John 1:8-10!
You will be blessed and comforted!