I have not spoken in secret from the beginning. God, "from the beginning," i.e. from his first dealings with Israel, had raised up a succession of prophets, who had declared his will, not "in secret," or ambiguously, but openly and plainly, so that all who heard might understand (comp.
Isaiah 45:19, and see the comment ad loc.). From the time that it was, there am I; i.e. "from the time that the earth was, there (in the succession of my prophetic messengers) was I." It was I who spake by their mouth, and thus announced my will publicly.
And now the Lord God, and his Spirit, hath sent me. Dr. Kay supposes that "one Divine Personage is here sent by another"—the Second Person of the Holy Trinity by the First and by the Third; but it is against the analogy of faith that the Third Person should send the Second.
Probably Mr. Cheyne is right in suggesting that "here a fresh speaker is introduced," and also right in his supposition that the fresh speaker is "the prophet himself," who tells us that he is now carrying on the goodly succession which has been "from the beginning," and is sent to deliver his message by God (the Father) and his (Holy) Spirit.
On the tendency of Isaiah to "hypostatize" the Spirit of God, see the comment on Isaiah 40:13; and compare Mr. Cheyne's note on the same passage.