Is everything that befalls a person good for him, even if it was a wrong decision he made? And is making a wrong decision considered part of divine decree (Qadaa' and Qadar)? And does this fall within the scope of "every matter of the son of Adam is good for him"?

All that occurs in the universe, be it good or evil, is by Allah's decree and destiny. A Muslim must be content and submit to what Allah decrees for him, for a calamity may, in its essence, contain a blessing. One must take the necessary means and be keen on what benefits, then entrust the matter to Allah the Almighty, and not dwell on the past by saying, "If only I had done such and such." The Istikhara prayer is recommended, as it is an entrustment to Allah to choose what is best. The one performing Istikhara should not have a pre-existing inclination or desire. It is advisable to seek another job and supplicate to Allah with humility, for Allah may have reserved something better.