What should I do about being rejected by a girl I loved and proposed to? And should I continue with Qiyam al-Layl (night prayers) and supplication to change destiny, knowing that I cannot tolerate anyone else?

Supplication enables a Muslim to achieve what they desire, and marriage is a preordained provision. Supplication does not alter what is written in the Preserved Tablet (Al-Lawh Al-Mahfuz), but it can change what is in the book held by the angel. If you were not able to marry the person you supplicated for, Allah may have reserved for you something better than what you asked for, because supplication is answered in one of three ways: either the supplication is granted expeditiously, or it is stored for him in the Hereafter, or an equivalent amount of evil is averted from him. So, if this girl refused, do not grieve over her, for good may lie in not marrying her. Entrust your affair to Allah and ask Him for marriage to a righteous woman.