One of the loneliest and most humane observations in the book.
Quick answer
Some of what you carry cannot be transferred. The verse states the limits of being known, and it does not scold anyone for those limits.
Read both lines
Both halves say the same thing about opposite feelings, which is what makes the verse fair rather than bleak. Grief has a private core nobody else reaches, and so does gladness; a person can be surrounded by people who love them and still be the only one inside their own experience. The word for what the heart knows is the bitterness of its own nephesh, its own life. Proverbs offers no remedy here and does not pretend to. It simply reports something true, and the reporting is itself a kind of company.
From this side of the cross
The verse marks a boundary that human friendship does not cross, and it leaves it standing. What the gospel adds is not a technique for being better understood but the claim that the boundary has been crossed from the other side: we do not have a High Priest "who cannot sympathize with our weaknesses, but was in all points tempted as we are" (Hebrews 4:15). That is not an argument against the proverb. A stranger still does not share your joy. Someone who is not a stranger does.
Related: Hebrews 4:15 · Proverbs 14:13 · Romans 12:15
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.