Will Allah hold me accountable for the bad thoughts that I resist but then succumb to without them turning into actions? And what is the form of seeking forgiveness (Istighfar) that relieves worries and eases affairs?

A person is not held accountable for such thoughts and notions, for Allah, in His mercy for this Ummah, has overlooked what they ponder within themselves, as long as they do not act upon it or speak it. One must continue to strive to get rid of them. The shortest forms of seeking forgiveness are: "Astaghfirullah" (I seek forgiveness from Allah) or "Rabbighfirli" (My Lord, forgive me). The best supplication for seeking forgiveness (Sayyid al-Istighfar) is: "Allahumma anta Rabbi la ilaha illa Anta, Khalaqtani wa ana abduka, wa ana ala ahdika wa wadika mastatatu, Audhu bika min sharri ma sanatu, abu'u laka binimatika alayya, wa abu'u bidhanbi faghfir li fa innahu la yaghfiru adhdhunuba illa Anta" (O Allah, You are my Lord, there is no god but You. You created me, and I am Your servant, and I am abiding by Your covenant and Your promise as much as I can. I seek refuge in You from the evil of what I have done. I acknowledge Your blessings upon me, and I acknowledge my sin, so forgive me, for none forgives sins except You).