John Gill Commentary Psalms 119:34

"Give me understanding, and I shall keep thy law; Yea, I shall observe it with my whole heart." — Psalms 119:34 (ASV)

Give me understanding, and I shall keep your law
A spiritual understanding; an understanding of the law, the perfection, purity, holiness, and spirituality of it; an understanding of the Gospel, and of Christ and the things of Christ; from whom grace and strength are to be had for the due observance of the law, as in his hands.

Which understanding must be given, and is a gift of pure, free, rich grace, to such who have it; though they cannot keep the law perfectly, as no mere man can, yet will keep it spiritually, from a principle of love and gratitude, and with a view to the glory of God and Christ, (1 John 5:20) ;

yea, I shall observe it with [my] whole heart ;
not only externally, and to be seen of men, and get applause from them; but doing the will of God from the heart, and with a good will and heartily, as to the Lord, and not to men; with a sincere affection for him, and with a single eye to his glory, (Ephesians 6:6Ephesians 6:7) (1 Timothy 1:5) .