What is the legal ruling in Islamic law regarding dealing with doubt about breaking one's intention due to obsessive-compulsive disorder, and how can a person afflicted with OCD ascertain the persistence of their intention without following obsessive thoughts?

The questioner's intense scrutiny in recalling the intention indicates the continuity of their intention. Any obsessive thoughts that occur to them are only from Satan. Islamic Sharia is built on facilitation and removing hardship. Allah Almighty says: "Allah intends for you ease and does not intend for you hardship" (Al-Baqarah: 185). The Prophet, peace and blessings be upon him, said: "Indeed, this religion is easy, and no one will ever make the religion difficult except that it will overwhelm him." The questioner's strictness upon himself stems from blameworthy obsessive thoughts that lead to distress and difficulty. The Prophet, peace and blessings be upon him, said: "Ruined are those who go to extremes" (narrated by Muslim). The best treatment for these obsessive thoughts is to ignore them.