What should I do after the girl I loved rejected my proposal and got engaged to someone else, which negatively affected my life?

Everything that befalls a person is by the decree of Allah, and this belief helps the believer endure calamities. The Prophet (peace be upon him) advised against saying "If only I had done such and such" when afflicted by calamities, but rather "Allah has decreed and what He wills, He does." Life should resume naturally after a specific marriage proposal has passed, for perhaps there was good in that. One should not remain in a state of isolation, abandoning people, food, drink, and work. Love in Islam has its regulations, just as the prohibition of one person proposing over another's proposal applies after the first proposal has been accepted, not before it.