Quick answer
The grief of verse 21 again, now including the mother. The verse names both parents and adds nothing to soften it.
Read both lines
Where verse 21 said this twice about a father, this verse divides it between two people and uses a different word for each: grief for the one, bitterness for the other. "Her who bore him" is a deliberately physical phrase rather than simply "his mother," and it puts the cost in the body of the person who carried him. The repetition four verses later is not an accident of compilation. Some sorrows recur in a life, and the book lets this one recur on the page.
Related: Proverbs 17:21 · Proverbs 10:1 · Proverbs 23:24–25
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.