Is a devotional act (prayer/ablution) interrupted merely by the thought of interrupting it, or is it considered complete due to the fulfillment of its pillars and obligations?

It is clear from the question that the inquirer is suffering from waswasah (obsessive whispering), and therefore, it is advised to disregard these whisperings and not pay attention to them. Delusion has no bearing; whoever experiences many whisperings and feels that worship has been interrupted should not pay attention to their delusion. As for one who intends to interrupt an act of worship, if their intention comes after completing it (such as after the tasleem in prayer or after pouring water over the entire body in ghusl), their interruption has no effect because the act of worship has already been completed. If they interrupt it before the completion of the act of worship, then if it was prayer, it becomes invalid and they must repeat it. If it was taharah (purification), they must renew the intention for the remaining parts of the taharah. Al-Suyuti mentioned that it is by consensus that one who intends to interrupt the prayer after completing it does not invalidate it. Al-Khatib Al-Shirbini clarified that intrusive waswasah that occurs to the mind without choice has no effect, and that whoever intends to interrupt wudu' has their intention severed and must renew it for the remaining parts.