Now this is a very important verse:

Romans 8:15 For ye received not the spirit of bondage again unto fear; but ye received the spirit of adoption, whereby we cry, Abba, Father. ASV

 You see, we can have the spirit of fear, or we can choose not to.  It is a choice we make.  If it was not a choice then the Lord would not command us not to be anxious.  The Lord does not command us to be, say, blonde or right handed or fast runners.  He commands only what we are able to accomplish.  Now we need Him to do these things, but He has already set it up for us to succeed.  We are the only ones stopping us from obeying.