studylight.org/commentary/matthew/26-39.html
I found this commentary here, which sheds some light on the meaning of the “if possible”.
“…the removal of the cup from Christ was possible in itself, but not as things were circumstanced, and as matters then stood; and therefore it is hypothetically put, “if it be possible”, as it was not; and that by reason of the decrees and purposes of God, which had fixed it, and are immutable; and on account of the covenant of grace, of which this was a considerable branch and article, and in which Christ had agreed unto it, and is unalterable; and also on the score of the prophecies of the Old Testament…”
Then, the commentary goes on to say : “Besides, Christ had foretold it himself once and again, and therefore consistent with the truth of his own predictions, it could not be dispensed with.”
So I am left with my question: knowing all this, why petition the Father? I don’t see another reason than for the reader to get a glimpse of the violence of His agony.