Why do we have a hard time resting?
Because we do not believe that He is working.
Yet He is at work!
"5God thundereth marvellously with his voice; great things doeth he, which we cannot comprehend.
6For he saith to the snow, Be thou on the earth; likewise to the small rain, and to the great rain of his strength.
7He sealeth up the hand of every man; that all men may know his work.
8Then the beasts go into dens, and remain in their places.
9Out of the south cometh the whirlwind: and cold out of the north.
10By the breath of God frost is given: and the breadth of the waters is straitened." (Job 37: 5-10)
Jesus our Savior, through whom the whole universe was created, still holds the worlds together, including ours:
"15[Jesus] is the image of the invisible God, the firstborn of every creature: 16For by him were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him: 17And he is before all things, and by him all things consist." (Colossians 1: 15-17)
During His earthly ministry, Jesus was always working:
"Jesus saith unto them, My meat is to do the will of him that sent me, and to finish his work." (John 4:34)
"16And therefore did the Jews persecute Jesus, and sought to slay him, because he had done these things on the sabbath day. 17But Jesus answered them, My Father worketh hitherto, and I work." (John 5: 16-17)
Notice that Jesus worked on the Sabbath Day, not only upending the traditions of the Pharisees, but signaling to all of us today that He is our rest, and He is working for us:
"For even the Son of man came not to be ministered unto, but to minister, and to give his life a ransom for many." (Mark 10: 45)
"20I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. 21I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." (Galatians 2: 20-21)
"12Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling. 13For it is God which worketh in you both to will and to do of his good pleasure." (Philippians 2: 12-13)
We rest in Him, and thus He is able to work in us.