When you think about Psalms is all about Gods love and what he is doing is showing us that love. You would have read the text to understand that these are songs. 136 is teaching us how to worship how give him praise and his mercy will be with us forever. You praise God you worship him and the enemy can not touch you. When you are with him and he is with you. This mercy is for the weak and need the power of God to come through and help you. When there is a flute of distress a mercy will come soon; come and take away the stress. You can make it with God. Don't give up because you will be strong in the end and the light will shine; you will feel so much lighter as in a cloud has lifted you up and your peace has flown in.
Don't be discouraged Isaiah42:4.
Have faith and it will all be revealed! ROMANS 1:17
If it's up to God his word is all powerful Hebrew 4:12
Love y'all have a good night!