Cast me not away from thy presence. To he "cast away from God's presence" is to be altogether cast out of his covenant, made an alien from him, deprived of his favour and the light of his countenance (see Genesis 4:14; 2 Kings 13:23).
The psalmist deprecates so terrible a punishment, although he feels that he has deserved it. And take not thy Holy Spirit from me. God's Holy Spirit had been poured upon David when he was first anointed by Samuel to the kingly office (1 Samuel 16:13).
His great sins had undoubtedly "grieved" and vexed the Spirit; and, had they been continued or not repented of, would have caused him to withdraw himself; but they had not "wholly quenched the Spirit" (1 Thessalonians 5:19).
David was therefore able to pray, as he does, that the Holy Spirit of God might still be vouchsafed to him, and not be "taken away," as from one wholly unworthy.