Mt 18:13 "And if so be that he find it, verily I say unto you, he rejoiceth more of that sheep, than of the ninety and nine which went not astray." (KJV)
A friend in prison told me that he didn't know if God could ever forgive him for what he had done. He has been remorseful for years, and he loves God; but guilt plagues him. He's much like many believers.
As a believer, I know that harboring sin in my life separates me from fellowship with God. (Psa 66:18). And I know from John 1 that if I deny that I have any sin, I am only deceiving myself. But John 1:9 says that if "we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness." (KJV) However, many believers are stuck with a guilt that paralyzes us. "I'm too much a sinner for God to forgive," people say.
Some will say (correctly) that if you won't believe that God forgives you, that is doubt, and doubt is sin. Ok... but it isn't much of a remedy for the believer stuck in despair. I expect that is just he "Chin up; get over it" advice that doesn't help.
But look at Mt 18. It is helpful from two angles. First is that the shepherd, a type of Jesus, rejoices that he found the sheep who went astray! Look at this picture. The Shepherd has rounded up his sheep, perhaps in a stable, but maybe just in a quiet place. He does a quick count - one sheep short. The Shepherd knows His sheep well enough to figure out which one is missing; sigh! it's Petey again! He thinks where Petey might have gone astray, and as the other sheep bed down, he heads back the trail retracing their steps. Much later, the Shepherd is tired and bone-weary, when he hears a bleat. Petey! He's stuck in a thorn-bush and can't get loose. The Shepherd gently untangles the sheep, picks him up and heads back to the herd. The next day, he sees a friend and tells the friend how glad he is to have found the wayward sheep. He's surely glad the other 99 didn't run off; but he is truly glad he found Petey!
Ok, if you're one of the 99, you might be peeved that the Shepherd didn't come back and say to all the sheep: "You 99 sheep are wonderful! You followed me and followed where I led, and you make my life easier. I'm so glad you aren't like Petey!" And why? Because he loved Petey as much as the rest!
So if you feel like Petey, remember God comes looking for you because He LOVES you. Because He loves you, He may discipline you, as the Shepherd did for Petey, so he would learn not to wander. But He loves you.
And if you feel like one of the obedient sheep, relax: God loves you, too; but not more than he loves the wayward one. And remember that in the parable of the Pharisee and the Publican, (Lk 18:10-14), "I tell you, this man [the publican] went down to his house justified rather than the other: for every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted." (KJV) This view won't help your ego, but it will help your spiritual walk.
Quotes from The King James Version, (Cambridge: Cambridge) 1769.