Yet have I trusted in thee, O Jehovah! The rendering properly is, And I have trusted in thee; but the Hebrew copulative particle v, vau, and, is used here instead of the adversative particle yet, or nevertheless. David, setting the steadfastness of his faith in opposition to the assaults of the temptations of which he has made mention, denies that he had ever fainted, but rather maintains, on the contrary, that he stood firm in his hope of deliverance from God. Nor does this imply that he boasted of being so magnanimous and courageous that he could not be overthrown through the infirmity of the flesh. However contrary to one another they appear, yet these things are often joined together, as they ought to be, in the same person, namely, that while he pines away with grief, and is deprived of all strength, he is nevertheless supported by so strong a hope that he ceases not to call upon God. David, therefore, was not so overwhelmed in deep sorrow, and other direful sufferings, as that the hidden light of faith could not shine inwardly in his heart; nor did he groan so much under the weighty load of his temptations, as to be prevented from arousing himself to call upon God. He struggled through many obstacles to be able to make the confession which he here makes. He next defines the manner of his faith, namely, that he reflected with himself thus that God would never fail him nor forsake him. Let us mark his manner of speech: I have said, Thou art my God In these words he intimates that he was so entirely persuaded of this truth, that God was his God, that he would not admit even a suggestion to the contrary. And until this persuasion prevails so as to take possession of our minds, we shall always waver in uncertainty. It is, however, to be observed, that this declaration is not only inward and secret - made rather in the heart than with the tongue - but that it is directed to God himself, as to him who is the alone witness of it. Nothing is more difficult, when we see our faith derided by the whole world, than to direct our speech to God only, and to rest satisfied with this testimony which our conscience gives us, that he is our God. And certainly it is an undoubted proof of genuine faith, when, however fierce the waves are which beat against us, and however sore the assaults by which we are shaken, we hold fast this as a fixed principle, that we are constantly under the protection of God, and can say to him freely, Thou art our God.

But I trust in thee, O Lord - In these times of trial - when Psalm 31:9 his eye was consumed with grief; when Psalm 31:10 his years were spent with sighing, his strength failed, and his bones were consumed; when Psalm 31:11 he was a reproach among his neighbors, and dreaded by his acquaintances; when Psalm 31:12 he was forgotten as a dead man; and when Psalm 31:13 he was surrounded with causes of alarm. Then he trusted in God. His confidence did not fail. He believed that God was his Father and Friend; that He was on the throne; that He could protect and defend him; and he left himself and his cause with Him. In such circumstances as these there is no other sure refuge but God; at such times the strength of faith is shown, and then is seen pre-eminently the power and value of religion.
I said, Thou art my God - Thou art all that is implied in the name "God;" and thou art mine. He felt assured that God would not forsake him, though men did; that he might confide in Him, though his earthly friends all turned away. There is always one (God) who will not leave or forsake us; and the friendship and favor of that One is of more value to us than that of all other beings in the universe combined.

But I trusted in thee - Hitherto thou hast been my Helper, and thou art my God; I have taken thee for my eternal portion.

But I trusted in thee, O LORD: I said, (k) Thou [art] my God.
(k) I had this testimony of conscience, that you would defend my innocence.

But I trusted in thee, O Lord,.... His faith revived again under all the discouraging views he had of things, and was exercised upon the Lord; he committed himself to him, believing he was able to help him in his time of trouble, and deliver him;
I said, thou art my God; he claimed his covenant interest in him, and used it as an argument with him to have regard unto him, and as a support to his faith in his present distress; See Gill on Psalm 7:1.

In his profession of trust he includes the terms of the prayer expressing it.

(Hebrews.: 31:15-19) But, although a curse of the world and an offscouring of all people, he is confident in God, his Deliverer and Avenger. By ואני prominence is given to the subject by way of contrast, as in Psalm 31:7. It appears as though Jahve had given him up in His anger; but he confides in Him, and in spite of this appearance, he even confides in Him with the prayer of appropriating faith. עתּות or אתּים (1-Chronicles 29:30) are the appointed events and circumstances, the vicissitudes of human life; like the Arabic 'idât (like עת from ועד), the appointed rewards and punishments. The times, with whatsoever they bring with them, are in the Lord's hand, every lot is of His appointment or sending. The Vulgate follows the lxx, in manibus tuis sortes meae. The petitions of Psalm 31:16, Psalm 31:17, spring from this consciousness that the almighty and faithful hand of God has mould his life. There are three petitions; the middle one is an echo of the Aaronitish blessing in Numbers 6:25. כּי קראתיך, which gives the ground of his hope that he shall not be put to shame (cf. Psalm 31:2), is to be understood like אמרתּי in Psalm 31:15, according to Ges. 126, 3. The expression of the ground for אל־אבושׁה, favours the explanation of it not so much as the language of petition (let me not be ashamed) of as hope. The futures which follow might be none the less regarded as optatives, but the order of the words does not require this. And we prefer to take them as expressing hope, so that the three petitions in Psalm 31:16, Psalm 31:17, correspond to the three hopes in Psalm 31:18, Psalm 31:19. He will not be ashamed, but the wicked shall be ashamed and silenced for ever. The form ידּמוּ, from דּמם, is, as in Jeremiah 8:14, the plural of the fut. Kal ידּם, with the doubling of the first radical, which is customary in Aramaic (other examples of which we have in יקּד, ישּׁם, יתּם), not of the fut. Niph. ידּם, the plural of which would be ידּמּוּ, as in 1-Samuel 2:9; conticescere in orcum is equivalent to: to be silent, i.e., being made powerless to fall a prey to hades. It is only in accordance with the connection, that in this instance נאלם, Psalm 31:19, just like דּמם, denotes that which is forcibly laid upon them by the judicial intervention of God: all lying lips shall be dumb, i.e., made dumb. עתק prop. that which is unrestrained, free, insolent (cf. Arabic 'âtik, 'atı̂k, unrestrained, free
(Note: But these Arabic words do not pass over into the signification "insolent."))
is the accusative of the object, as in Psalm 94:4, and as it is the nominative of the subject in 1-Samuel 2:3.