From In the Wilderness of Sinai by Rabbi Avigdor Miller
Q: “Ovar v’shana nasis lo k’heter” (Shaarei Teshuva): if a man does a sin more than once, then it becomes to him like permissible, it means he loses his feeling of fear of the sin. So the question is, would teshuva (repentance) be able to accomplish a regeneration of his attitude, that he should regain the fear of the sin?
A: That’s what teshuva implies. Teshuva means you look back and you want to undo the impression of the past. If a man did a sin in his youthful years, and now he stopped doing that sin, but whenever he recalls that sin he smacks his lips for the good time that he had, he thinks it was a good time that I had then, oh no, that’s not at all teshuva.
A baal teshuva must shudder in disgust at the wicked thing he has done
A baal teshuva (penitent person) has to think, at that time I ate from a dead rat. Let’s say he had an encounter with a pretty girl, when he looks back on it, his sentiment should be, I was eating a dead rat. That's how the teshuva is. He has to have such remorse that he dirtied himself by eating a dead rat that was ripening for a few weeks in the July sun. Suppose he would approach that rat again; he would have to cross the street. That’s what he would do now, if he would encounter that girl again and he would be still in his full youthfulness again, he’d run across the street and hold his nose.
That’s a baal teshuva. He has to be so sorry at the foolish thing that he did, that he corrupted himself and he ruined himself, that all of his life whenever that episode recurs to his mind, he shudders in disgust at the wicked thing he had done.
Now that’s not easy, of course. If a person can stop doing the sin, and that alone, that’s also something. But charata (regret) is one of the conditions of teshuva shlemah (complete repentance).