Is what happened a destiny from God, and He does what He wills, or are wrong actions the cause? And should one say "God has decreed and He does what He wills" after attempting the right action and it fails?

A Muslim must take the available means, and if he falls short, he is blameworthy. If he takes the means and then something overcomes him, he should say: "Qaddar Allah wa ma sha'a fa'al (Allah has decreed and what He willed, He did)." Sheikh al-Islam Ibn Taymiyyah clarified this principle, citing the Prophet's (peace and blessings be upon him) hadith: "Indeed, Allah blames for incapacity, but you must be resourceful. If something overcomes you, then say: 'Hasbi Allah wa ni'ma al-wakeel (Allah is sufficient for me, and He is the best Disposer of affairs),'" and the hadith: "The strong believer is better and more beloved to Allah than the weak believer, and in both there is good. Be keen on what benefits you, seek Allah's help, and do not be helpless. If something overcomes you, do not say: 'If only I had done such and such, it would have been such and such,' but say: 'Qaddar Allah wa ma sha'a fa'al (Allah has decreed and what He willed, He did),' for 'if only' opens the door to the work of Satan." So, a person is between two matters: a matter he is commanded to do, for which he must be keen, seek Allah's help, and not be helpless; and a matter that befalls him without his action, for which he must be patient and not despair. As some wise people said: "Matters are two: a matter for which there is a solution, so do not be incapable of it; and a matter for which there is no solution, so do not despair of it."