What should a girl do about recurrent sexual thoughts and whisperings, constant doubt about the emission of manī (seminal fluid), repeated ghusl (ritual bath) due to this doubt, and the effect of images she sees by mistake, given her prior knowledge of the rulings regarding manī and madhy (pre-seminal fluid)?

Your imaginings and thoughts do not harm you as long as you strive against them and do not succumb to obsessive whispers. You are not required to perform ghusl (definition) (ritual bath) except when semen is discharged. Vaginal wetness is pure but invalidates wudu (ablution). If you doubt what has exited, you may choose between the ruling of semen, pre-seminal fluid, or other discharges.