In Psalm 139:23-24 it is telling God to search me and try me. That takes trust to give God the reins and let him examine every dark corner of my heart. To find all the things that he doesn't want there and purify my heart. That can be a scary place to be. A very vulnerable place. Who wants to be exposed and raw like that?
I know God already knows me and my thoughts so why do I want to even hold back? He is not surprised by what he finds. It's just me that cringes when his light and truth expose the places I tried to make secret. Things I don't want to look at are coming to the surface, but I am not met with judgement and disdain from God. I am met with his healing love instead. His Holy Spirit is gently and lovingly leading me on. He knows it is for my good and not for my harm to bring me to full healing.
The rest of Psalm 139 is wonderful to read to remind myself exactly who I am to God. When I get to a dark place in my heart that I don't want to look at, I can read that I am fearfully and wonderfully made. Marvelous are God's works, including me. It says that I am on God's mind and many are his thoughts toward me. It says he knows everything about me and is with me everywhere, even in the dark it is as light to him. Nothing is hid. His hand is upon me and I cannot go anywhere where he is not. He will lead me no matter where I go.