Mission completed
When discouraged and heavy thoughts creep into our soul.

We build our hope on what God has promised.

The main theme of that section (Ps 119:49-56), which now takes up the Hebrew letter ZAYIN, is the comfort the psalmist finds in God's word when the culture he lives in mocks and persecutes those who believe in God's truths. He has full confidence that God will save him, because he rests in the Lord's law.

Psalm 119.49 Remember your word to your servant. Through it you have given me hope.

This section does not start unexpectedly with a prayer to the Lord. David has learned to pray. He knows that the Lord hears when he prays. In this stanza he reminds God of the promises he has received from the Lord: Remember the Word you have given your servant! Don't forget to do what you said! This is a very important lesson to learn.
Firstly, we must learn to absorb the Word so that it becomes personal to us. Or put another way, we must learn to hear God's voice when we read and base our thoughts on the words of the Bible. It is possible to read the Bible without hearing God's voice, without the Word speaking to us. We must avoid that at all costs.
I am completely convinced that the Bible is the word of God. Nevertheless, it can happen that I read my daily chapters without something coming alive for me, without something speaking to me. Then I have to stop. Then I must humbly ask the Lord for help to get hold of what he wants to tell me in what I have read. Then I read once more while praying that I notice something I can learn from what I read. As a rule, God hears that prayer and I see things I didn't notice the first time I read.
Many good promises in God's word have become my personal words from God. But of course I get words from the Lord in other ways also. It is the same how God's word comes to us, when we have received a promise from God, we must constantly remind God of what he has promised.
Note the next stanza: Through it you have given me hope! When God speaks to us, we gain hope. Through the Word we get something to hope for, something to look forward to. The promises of God create light in the dark. Words from God drive the dark and heavy thoughts away, because they create hope.
The good thing is that God is a faithful God. He keeps what he promises and does what he says he will do. It doesn't matter how difficult life can be, God will never leave us or let us go. When we remind God of what he has said, it also helps us. It helps us hold fast to the promises and to the hope. It helps us to fix our eyes on the faithful God who keeps what he promises. We build our hope on what God has promised.
Lord, don't forget the promises you have given me, because they are my hope and courage!