17 “Save Me from the mire, that I may not stick”. From that whereof above he had spoken, “Fixed I am in the clay of the deep, and there is no substance.” Furthermore, since you have duly received the exposition of that expression, in this place there is nothing further for you to hear particularly. From hence he says that he must be delivered, wherein before he said that he was fixed: “Save Me from the mire, that I may not stick.” And he explains this himself: “Let Me be rescued from them that hate Me.”
They were themselves therefore the clay wherein he had stuck. But the following perchance suggests itself. A little before he had said, Fixed I am; now he says, Save Me from the mire, that I may not stick: whereas after the meaning of what was said before he ought to have said, Save Me from the mire where I had stuck, by rescuing Me, not by causing that I stick not. Therefore He had stuck in flesh, but had not stuck in spirit. He says this, because of the infirmity of His members.
Whenever perchance you are seized by one that urges you to iniquity, your body indeed is taken, in regard to the body you are fixed in the clay of the deep: but so long as you consent not, you have not stuck; but if you consent, you have stuck. Let then your prayer be in that place, in order that as your body is now held, so your soul may not be held, so you may be free in bonds.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)