Why are you cast down, O my soul? And why are you disquieted within me? Hope in God, for I shall yet praise Him, for the help of His countenance – Psalm 42:5
How often do you speak to yourself? While the world would make us think that speaking to yourself is weird, this verse of the bible shows us that it is perfectly normal to speak to ourselves. We should speak to ourselves anytime our thoughts are out of sync with God’s thoughts. We should speak to ourselves when our mind, will, and emotions are not in sync with our true position and identity in Christ. The bible tells us that David encouraged himself (1 Samuel 30:6). To encourage means to give courage. In life, we must make it a practice to give ourselves some courage and strength in the Lord when our soul is downcast. Remember that you are not your soul! You are a spirit-being that possesses a soul. That means your soul is your property and so can take instructions from you. Just as David spoke to his soul, you too should speak to our soul. Make it a practice to speak the truth to your soul about who you are, who God is to you, and what you have in Him. It will make a big difference in how your days go.
My soul is not in control of me; I am in control of my soul
SuperChamp 
I have often read this verse as being upset when something massive happens, which is part of it but not everything.
It reminds of the fact that thoughts inform how I feel and subsequently act,which has consequences.
So it starts with what I’m thinking. It doesn’t matter how realistic my thinking is or how steeped in facts, if it is contrary to the word of God, I need to take my thoughts captive – cast down those thoughts -and make it obedient to Christ.
Secondly, whilst I will have emotions as a human being, I don’t have to allow them to be the boss of me when my feelings are not aligned with my true position and identity Christ.
I like the reminder of the position of the soul and being empowered by the truth that I can instruct my soul with the word of God. I do not have to be subject to it’s whims.
A practical way that is being highlighted for me to work this out is watching what I say about myself. Learning to be quiet when it is negative, even when I believe it is just being realistic. Instead, practice learning what the Bible says about the situation and speaking that instead.