We have been instructed by James to submit to God and resist the devil. In this verse, he encourages us to respond to our sinfulness emotionally. He encourages us to cry, to mourn, to even be gloomy. Despite laughter and joy, he encourages us to set them aside. It is not James' intention to suggest that we accept sadness as an ongoing way of life. Rather, Christians should be known for their joy.
As Christians, we are not to rationalize sin but to lament its destructive nature both in our own lives and in the lives of others. We are told by Jesus Himself that those who mourn and weep over their sin and the harm it causes will be blessed. God is blessing those who mourn, for they will be comforted. Godly sorrow produces repentance, which results in salvation.
While lamenting over sin is important, it should not be construed as wallowing in the wretchedness of our sin in an effort to 'justify' ourselves to God. We are all responsible for admitting our sins and grieving over the devastation caused by them. Through faith in Christ, a sinner can find reconciliation with God and forgiveness through weeping over sin.

Dear Heavenly Father, the book of James gives us wise instructions on how to live godly and victorious lives. Thank You for giving me everything I need for life and godliness through faith in Jesus Christ. Keep me close to You, and help me surrender my life to You. Help me keep a pure heart and avoid double-mindedness. It is my prayer that I hate sin in all its forms and grieve over those things that dishonor Your holy name. 
In Jesus's name I pray. Amen.