there are some verses in Psalm 119 that prove it is the Son's prayer for life because of His obedience to the law and the promise of life in the law to Him. those verses preclude it from being anyone else's prayer. one of the main ones are verse 106 and 107
106. I have sworn, and I will perform
it, that I will keep thy righteous judgments.
107. I am afflicted very much: quicken me, O YHVH, according unto thy word.
He took an oath and confirmed it to keep His Father's judgments which is the Hebrew word mispat H4941. you can look up that word and see where it is used in other verses. One place it is used is in Leviticus 18:5 which is the promise of life in the law to Him
Leviticus 18:5 Ye shall therefore keep My statutes, and My judgments:
H4941 which if a Man do, He shall live by them: I
am YHVH.
So the Son took an oath to keep His Father's judgments which is an oath to keep the law. no one else could pray that prayer. He is the only One Who kept the law which required Him to give His life to save us. He also praises the Torah throughout Psalm 119 which was His path to life and being raised from the dead.
in verse 107 the affliction He was speaking of was death. the word quicken is haya H2421 again. He is asking His Father to raise Him to life again after giving His life to save us in obedience to the law and keeping His Father's judgements. It was His Father Who required Him to offer His life for us. His Son performed it.