Trusting my all to his tender care

So, I have an important exam I have to take at the end of the month and I am worried.  I hate standardized exams!  I would much rather prove my knowledge in a written or oral assignment.  Anyways, I am studying for this exam as often as possible and trying not to worry.  Yesterday, I was prompted with two thoughts:

 "...for your Heavenly Father knoweth that ye have need of all these things. Take therefore no thought for the morrow: for the morrow shall take thought for the things of itself. " (Mattew 6: 32, 34)

"So trusting my all to thy tender care,
And knowing thou lovest me," 
(Hymn 270)

Why is it so hard to trust Heavenly Father with everything?  I have done this lesson before, yet here I am again.  He knows me.  He knows my needs.  He knows my concerns.  He knows my hopes and dreams.  And he knows yours---so let's trust him and give no thought for our tomorrows but do well today.