Is it obligatory to perform ghusl and repeat prayers and fasting if one thinks about sex and marriage without specifying a particular person, and a fluid is discharged, of which I do not know whether it is semen or madhiy, taking into account that I was not aware of the Sharia rulings, and what is the ruling on the whispers (waswas) that afflict me concerning my purity, prayers, and fasting?

Your prayers and fasting are valid, and you are not required to repeat any of them. The emission of madhy (pre-seminal fluid) or mani (seminal fluid) due to thoughts does not invalidate fasting according to the majority of scholars. If you are uncertain whether the discharge is madhy or mani, you may consider it either. You are not required to perform ghusl (ritual bath) if you consider it madhy, and your prayers are valid. We advise you to desist from these whisperings (waswas).