Before you begin:
Here, in Matthew 16:21-23, Jesus is telling (showing, it says) his disciples that he has to suffer a great deal, and then be killed, and then he'll rise again on the third day. Peter is just in plain denial at this awful news, from what I read, and he's rebuking Jesus -- he's speaking sharply to Jesus, criticizing him, basically telling Jesus he's wrong. Peter says, "This shall never happen to you."
And Jesus says to Peter, "Get behind me, Satan! You are a hindrance to me. For you are not setting your mind on the things of God, but on the things of man."
What I Took Away:
Ultimately, we do not have power over the outcome of our situation, if we're looking at the very broad, big picture. But we have power over our thoughts, our attitudes, our actions and reactions, and our hearts in the process.
God has the ultimate power over every circumstance, but you have the power to change your circumstances merely by changing the way you view them.
Peter responded so harshly to Jesus, that it was clear to Jesus that Peter had been temporarily overcome by Satan, which was why he had given in to such doubt. Peter wasn't going to change the outcome of the situation by lashing out at Jesus, though! (And we won't either.)
Jesus took control of the situation, demanded that Satan "get behind" him, and told Peter to set his mind on the things of God. When we set our mind on the things of God, we can rest easy. Sure, there will be very, very hard times for some of us, and of course we each won't have a good day every day, but if we take a moment to look at the big picture and realize that God's got this, we will find ourselves at peace. (Also see Daniel 3.)
What is something that you need to let go of? What is something that you need to change your attitude about? What is something you need to have a change of heart over? When things go badly for you, how do you react? What are your actions in sad circumstances?
You have power over your thoughts, your attitude, your actions and reactions, and your heart through whatever it is you're going through. Look to God. Give it to Him. Know that He has your best interest in mind, even if you can't see it.