2 Corinthians 7:10
“For godly sorrow produces repentance to salvation, not to be regretted; but the sorrow of the world produces death.”
There is a godly type of sorrow and an ungodly type of sorrow. Godly sorrow leads to repentance. Ungodly sorrow, or the sorrow of this world, just kills.
Our culture has rejected all "negative" emotions. However, God gave us the capacity for these negative emotions, and there is a proper use of them. Ec 7:3 says, "Sorrow is better than laughter: for by the sadness of the countenance the heart is made better."
We should feel bad about sin. There should be sorrow over our failures. However, this sorrow should lead to repentance, and then when forgiveness is received, our sorrow should be cast upon the Lord (Isa 53:4). Peter's sorrow after his denial of Jesus changed him, and certainly, he never regretted those tears he cried.
But the sorrow experienced by those who do not turn to God produces only death. They grieve over their situation because they don't turn to God (that's repentance). The mere sorrow of the world leads only to remorse and despair, to the death of a broken heart, possibly to suicide; in any case, to the loss of the true eternal life.
Christians should only have sorrow until they repent. Once repentance has come, we need to appropriate the forgiveness and cleansing that are already ours through Christ.
The Truth
“If we confess our sins, He is faithful and just to forgive us our sins and to cleans us from all unrighteousness.” (1 John 1:9)