Quick answer
Self-trust is named as folly outright. The verse is the counterpart to the verse before it, which named what to trust instead.
Read both lines
Placing this immediately after 28:25 is the chapter's argument: one trusts the LORD, the other trusts himself, and the same verb בּוֹטֵחַ opens both. So the two verses are one comparison split across a line break, and the label fool is unusually blunt for a book that normally describes rather than names.
The reason it is folly rather than arrogance sits in what this book means by the heart: the seat of judgment and desire, which 20:9 said nobody can call clean and 21:2 said God alone weighs. A man trusting his heart is not trusting a compass, he is trusting the instrument the book has spent chapters saying is unreadable from inside.
Related: Proverbs 3:5–7 · Proverbs 20:9 · Proverbs 21:2 · Proverbs 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.