How do I let go of an adult child and entrust them to God?

Say it as a handover, not a surrender. There is a difference between giving up on someone and giving them over. Name them, name the exact fear you carry, and then say out loud that you are placing them in God's hands, because your hands are too small and always were. Expect to do this again — some parents do it daily for years. The point is not to stop loving them; it is to stop believing your worry is what keeps them alive.

The act has a name: mesirah — a handing over, an entrusting: not abandoning what you love, but returning it to the One whose it already is.

From the sources “Cast your burden on the Lord, and He will sustain you.” — Psalm 55