Ps 35 is a desperate request by David for two things as listed in the first three verses:
1 Contend with my opponents, O LORD; fight against those who fight against me.
2 Take up Your shield and buckler; arise and come to my aid.
3 Draw the spear and javelin against my pursuers; say to my soul: “I am your salvation.”
Note that David requests two things of God:
- for God to fight his battles for him
- for comfort and assurance in his time of need by the LORD saying, "I am your salvation"
David goes on to say, if he is given these two things:
9 Then my soul will rejoice in the LORD and exult in His salvation.
10 All my bones will exclaim, “Who is like You, O LORD, who delivers the afflicted from the aggressor, the poor and needy from the
robber?”
That is, David recognizes his great need of divine intervention and comfort and pleads with God to provide both. When God does provide both, David rejoices in the LORD his God.
"Say to my soul ..."
In verse 3 we have the very Hebraistic idiom of using "my soul" as "me/I"; eg, Isa 42:1, Ps 43:5, 63:1, 116:8, etc. Thus, Barnes correctly observes:
Say unto my soul, I am thy salvation - Say to "me," I will save you. That is, Give me some assurance that thou wilt interpose, and
that thou wilt guard me from my enemies. Man only wants this assurance
to be calm in respect to any danger. When God says to us that he will
be our salvation; that he will protect us; that he will deliver us
from sin, from danger, from hell, the mind may and will be perfectly
calm. To a believer he gives this assurance; to all he is willing to
give it. The whole plan of salvation is arranged with a view to
furnish such an assurance, and to give a pledge to the soul that God
"will" save. Death loses its terrors then; the redeemed man moves on
calmly - for in all the future - in all worlds - he has nothing now to
fear.