Part of the fool run. Quoted in the New Testament at 2 Peter 2:22.
Quick answer
The image is disgusting on purpose. The verse is about the returning, the loop a person keeps completing, and not about the person's worth.
Read both lines
Ancient dogs were scavengers rather than pets, so the picture carried no affection to soften it, and the saying wants the reader's revulsion. What it is describing precisely is a return: the fool has been through this, it went badly, and he comes back to it. The Hebrew verb in the second line means to repeat or do again, so the folly is not new information to him.
Peter quotes it as "the true proverb" and pairs it with a washed sow going back to the mire (2 Peter 2:22). He is describing teachers who knew better, which is the same emphasis the proverb has: the horror is in the knowledge, not the appetite.
What it does not mean
This verse is not a label to hang on a person who has relapsed, and it is one of the easier verses in the book to use cruelly. Proverbs is describing a settled pattern in someone who will not learn, not the experience of a person fighting something that keeps beating them, and the difference between those two is enormous and usually invisible from outside. Addiction, compulsion, and the ordinary Christian experience of a besetting sin are not what this saying has in view, and someone grieving their own repetition is by definition not the figure being described. The fool of this chapter is untroubled. If a verse about a dog makes you ashamed of yourself, it is almost certainly not about you.
Related: 2 Peter 2:22 · Proverbs 27:22 · Proverbs 26:12 · Proverbs 17:10
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.