Does supplication change Allah's decree regarding marriage to a specific person, and does persistence in supplication achieve what is desired? And is there a correct way to seek forgiveness and make supplication, especially if it causes exhaustion and detachment from reality?

It is permissible for a woman to supplicate to Allah to grant her a specific person if she knows of his righteousness and religious commitment, for Allah is near and responsive. She must, however, be content with what Allah has decreed for her, and supplicate to Him to choose for her what is good and righteous in this world and the Hereafter, and to grant her a righteous husband. As for dedicating a session for the remembrance of Allah (dhikr) and seeking forgiveness (istighfar), that is something desirable and among the manners of dhikr and supplication.