Is the questioner considered an infidel, and must he repent of all sins for Allah to accept his repentance? And what is the difference between this infidelity and blasphemy against the Divine Being? Is the questioner excused because of his illness? And is it true that Allah seals the heart of someone like him, and that he is among the people of Hellfire, as stated in the verse: "Indeed, those who believed then disbelieved, then believed then disbelieved, then increased in disbelief - never will Allah forgive them, nor guide them to a way"? And what must he do to avoid these thoughts?

You are not required to repent from disbelief because you are not a disbeliever. Rather, what is required is repentance from masturbation. Your question about the difference between the disbelief of intrusive thoughts and the disbelief of cursing is unfounded, because you have not fallen into disbelief at all, and you are excused for these intrusive thoughts. Your refusal to marry is a mistake and a manipulation by Satan, so rely on Allah and accept what benefits you. Your citing of the verse: "Indeed, those who believed, then disbelieved, then believed, then disbelieved, then increased in disbelief" is not in its proper context, for the verse refers to those who persisted in their disbelief until they died, and the repentance of a disbeliever before their death is accepted, and a disobedient person is more worthy of acceptance. Protect yourself with remembrances (adhkar), the 'Mu'awwidhatayn' (Surahs Al-Falaq and An-Nas), and Ayat al-Kursi.