Is it permissible to express remorse by saying, "Would that I had not committed such-and-such a sin," or must one say, "God has decreed, and what He wills, He does"?

It is permissible for a Muslim to say: "I wish I had not done such-and-such" regarding what they dislike doing, and "I wish I had done such-and-such" regarding what they like doing. The word "لَيْتَ" (layta) is a particle of wishing, and it is permissible to use it for wishing in matters where wishing is allowed. Similarly, "لَوْ" (law) may sometimes convey the meaning of "لَيْتَ". As for saying "Allah has decreed, and what He willed, He did" out of regret and remorse for committing a sin or missing an act of obedience, it is legislated and falls under the category of citing divine decree as a response to a misfortune, not a sin. It is permissible to cite it after repentance and regret. However, citing divine decree as an excuse for committing a sin is forbidden.