Have you ever tried to run your car on empty? It can't be done.
Have you ever tried to go an entire day without food just because you forgot to eat - being too busy to take the time to fuel up your body?
Have you ever been so busy in God's work that you ran out of spiritual fuel?
A few weeks ago I was to that point - I was exhausted spiritually - drained by all that God was doing in and through me. Then I read a page from Oswald Chambers, My Utmost From His Highest, from February 9th. Here is an excerpt from that day. I hope you will take time to read the entire thing on your own. It really helped put things in perspective for me.
[Whether or not you experience exhaustion will depend on where you get your supplies. Jesus said to Peter, "Feed My sheep," but He gave him nothing with which to feed them. (John 21:17)
The process of being made broken bread and poured out wine means that you have to be the nourishment for other people's souls until they learn to feed on God. They must drain you completely --- to the very last drop. But be careful to replenish your supply, or you will quickly be utterly exhausted. Until others learn to draw on the life of the Lord Jesus directly, they will have to draw on His life through you. You must literally be their source of supply, until they learn to take their nourishment from God. We owe it to God to be our best for His lambs and sheep, as well as for Him..... You have no right to complain, "O Lord, I'm exhausted." He saved and sanctified you to exhaust you. Be exhausted for God, but remember that He is your supply. "All my springs are in you" (Psalm 87:7)] [emphasis mine]
Many times God calls us to "be still...." when it's the last thing we want to do. But it may be so that He can replenish us in His word by His Holy Spirit so we will have more to pour out the next time. It's OK to become exhausted as long as we take time to 'be still' and get replenished by our spiritual supplier - Jesus Christ!
John 15:5b [Jesus said] ...apart from Me you can do nothing.
Philippians 4:13 I can do all things through Christ which strengthens me.
1 Peter 5:10 And the God of all grace, who called you to His eternal glory in Christ, after you have suffered a little while, will Himself restore you and make you strong, firm and steadfast.