What should a girl do who realized, two years after reaching puberty, that the emission of something from her with desire necessitates ghusl, and she had not been performing ghusl before? She does not know the number of prayers and days she fasted in Ramadan without being in a state of purity, even though she performed ghusl after menstruation and made up two weeks of prayers?

You are not required to make up the prayers you performed without purification, because you were excused due to ignorance. This is the apparent ruling if it did not occur to you that ghusl was obligatory. It is more prudent to make up these prayers, but if they are so numerous that making them up becomes difficult, then there is no blame upon you for not doing so, and you should perform many supererogatory prayers. As for fasting, you are not required to make it up because purity is not a condition for it. The obsessive whispering (waswasah) you mentioned is a serious matter, so praise Allah Almighty for the blessing of Islam and ask Him for steadfastness, and devote yourself to acquiring beneficial Islamic knowledge.