The day Jesus was born, you may recall hearing about an angel who told the shepherds, "Do not be afraid... I bring you good tidings of great joy which will be to all people. For there is born to you... a Savior, who is Christ the Lord." And then as the narration continues, a whole host of angels suddenly appeared, "praising God and saying: 'Glory to God in the highest, and on earth peace, good will toward men!'"
What the angels were doing was proclaiming some exceptionally good news! Then one day, approximately thirty years later, John the Baptist proclaimed the good news as well. He saw Jesus coming toward him and said, "Behold! The Lamb of God who takes away the sin of the world!" So what we've got here is a Savior who came to take away the sin of the world. It's really good news all around, isn't it!
Not too long after this, Jesus came "preaching the gospel (good news) of the kingdom of God, and saying, 'The time is fulfilled, and the kingdom of God is at hand. Repent, and believe the good news.'"
This is where the meaning of the word "repent" is critical. In Greek it's the verb "metanoéō" (and the noun form of the word, repentance, is metánoia) and it means "to think differently or afterwards; to reconsider," or "a change of mind." That's all the word means. We have mistakenly read into the word to make it mean a change of behavior. But what Jesus is asking of people is simply this: Reconsider what you've always believed about God, and instead simply believe the good news! If you've thought of God as being against you, or being angry with you or out to strike you with a lightning bolt whenever you mess up, then repent of that thinking and instead believe that He is for you, He isn't angry with you and He's come to cure the world of the sin disease, rather than punish us.
"Repent" doesn't mean to be "sorry for your sin." It doesn't mean to "stop sinning and turn around and do what is right." Is it a good thing to stop doing things that "are not helpful" and that "don't edify?" (1 Cor 6:12, 10:23). Of course! But that's not what repentance is. Again, repentance means that we stop thinking wrongly about God as being angry with us and out to get us, and instead believe the good news that He took away your sin! He so loved the world that He gave His only begotten Son, that whoever believes in Him shall not perish but have eternal life!
If you think God is on your case about anything you've done, or the life you've lived, stop it! Take a look at the love God showed all of mankind on the Cross. "While we were still sinners, Christ died for us." There's no greater love. Change your thinking. Repent and believe the good news!