"Keep me as the apple of Your eye;
Hide me under the shadow of Your wings"
You are the apple of God's eye!
The apple or pupil of the eye is a most precious part of the body. To lose sight during David's time was to lose the possibility of being a productive member of the culture of society. David is saying that God will preserve me, defend me, and keep me as most precious and valuable!
I know there are times when you don't feel that way but your feelings should never determine how you believe God feels about you. God's Word should always be the determining factor of what you believe never your feelings.
Your are precious to the Lord, precious enough for Him to always defend you, protect you, and guard you!