Nothing gut-punches the soul of a Christian like a willful sin. I know all sins are willful to some degree. But there are those that are willful, and those that are really, really willful.
The shame that comes rolling into your conscience after you cuss at a person who cuts you off in traffic doesn't feel quite as gut-punchy as that which comes after you've watched porn or gotten drunk. Why? Because speaking improperly during a moment of fearful rage is more spontaneous than it is willful. Is it bad that was the first thing to come out of your mouth? Yes. But did you say to yourself when you got into your car that morning, "If someone cuts me off in traffic, I'm definitely going to drop the F-bomb on 'em"? No (at least I hope not). In the heat of the moment, without much thought, it just came flying out of your not-so-sanctified mouth.
But committing the other sins I mentioned requires a series of willful decisions to be made. You don't feel tempted to watch porn and then do it within less than a second. You have to decide to get alone by yourself, open your browser, type in that web address, and click that video (and the next one, and the next one, and the next one). You don't feel tempted to get drunk and then — whazam! — you're drunk. You have to decide to get dressed, order an Uber, walk into the bar, and order that drink (and the next one, and the next one, and the next one). At any point in the sequence of decisions, you can say, "no, I'm not doing this" or "no, I'm not going take this any further." You have so many chances to stop and turn back.
Therefore, if you end up watching porn or getting drunk, you said a whole bunch of yeses to get there. And if you're a Christian, this knowledge hurts worse than any hangover headache. Your heart sinks lower and your eyes grow wetter as you ask yourself why you chose to do this sin again, remember all the points at which you could have stopped but stubbornly refused, and mourn that you dishonored and grieved the God who gave his life for you. And don't let any well-intentioned person try to tell you, "Oh, don't beat yourself up, baby" — it is good and right that you feel this way! Grief is 100% appropriate.
However, the appropriate kind of grief, which the Bible calls godly grief, doesn't stall out but instead "produces a repentance that leads to salvation without regret" (2 Corinthians 7:10). When we commit a sin — however heinous and grievous that sin might be — it isn't pious to wallow in shame and despondency. That's called worldly grief, and it leads not to salvation but to the debilitation and destruction of faith. God may allow a dark cloud of painful emotions to settle over you for a little while — a few hours, a day, a week. He disciplines those whom he loves. But the God of the Bible is not one to smack you around with his belt day after day for the sin you committed last month or last year or in 1996.
No, he's one to speak gracious words of forgiveness and hope:
"Do not be afraid; you have done all this evil. Yet do not turn aside from following the Lord, but serve the Lord with all your heart. And do not turn aside after empty things that cannot profit or deliver, for they are empty. For the Lord will not forsake his people, for his great name's sake, because it has pleased the Lord to make you a people for himself" (1 Samuel 12:20-22).
God's desire is not that you'd try to feel as terrible as you can for as long as you can after every wicked thing you do. Your prolonged emotional suffering won't make you any cleaner. The blood of Christ alone makes you clean. God's desire is that you'd confess your sin, appropriately grieve your sin, trust him to be as gracious as he says he is, and keep following Jesus.
So by all means, feel the shame. Feel the regret. But then turn your eyes away from yourself, trust God to forgive you (yet again!), and get back up.
"For the moment all discipline seems painful rather than pleasant, but later it yields the peaceful fruit of righteousness to those who have been trained by it. Therefore lift your drooping hands and strengthen your weak knees, and make straight paths for your feet, so that what is lame may not be put out of joint but rather be healed" (Hebrews 12:11-13).