How can one be sure that they are not afflicted by sorcery, what is the proper way to deal with obsessive thoughts related to it, is Ruqyah for prevention or for treatment, and is it permissible to perform Ruqyah on oneself?

What you are feeling is closer to obsessive whispers (waswasah) due to what has been mentioned about magic. To cure it, you must resort to Allah by supplicating Him, consistently performing acts of obedience, staying away from sins, and frequently reciting the Quran, especially Surat Al-Baqarah or Ayat Al-Kursi. You can also perform Ruqyah on yourself with the aforementioned supplications and verses, maintain the morning and evening remembrances, and frequently engage in the remembrance of Allah.

It is impermissible to accuse anyone of magic without clear evidence, as Satan strives to sow doubt among people. If magic is presumed to be present in something, merely discarding it is not enough; it requires someone specialized to destroy it.