You can read the NIV translation of Psalm 21 here.
This is an interpretation.
God; as royalty, holy and chosen by you to reign with the heart of a servant, I rejoice with the strength that pours out from your presence in me – I have great joy, walking every day in the victories that you have laid out before me! You know each and every one of my hearts’ desires and you have given them to me in your grace – you hear every prayer that I could ever pray, and you have already said yes and amen.
You meet me, continuously, with blessings bigger and more extravagant than I could ever ask or imagine, and you put a crown on my head, so that I remember who you created me to be. I once asked you to fill my life, and of course you did; you have given me life, and life to the full – life that is full of your joy and your strength, forever.
You have already won the battles for me, and my load is light – I simply get to enjoy the glory of standing in your presence. Your blessings for me are great; I am filled to the brim with joy in your presence, and I can trust in you, for the love you have for me never fails and cannot be shaken.
When I put my foot on the battle field, you go before me and my enemies are defeated before I can even breathe a word. You will deal with them, and the harm that they bring, for me; you burn them up before my eyes. Though they think they are greater than you and plot their harm and destruction, they cannot succeed while you are here: you are my guard, my protector, my shield.
Be lifted high my God, my Father and my protector; in your strength and your might you save me and I will forever sing of your great love.