What is intention, and how do I know that I have formed an intention? Does ablution (wudu), prayer (salat), ritual bathing after major impurity (ghusl from janabah), and embracing Islam become invalid if I interrupt the intention or doubt it? Is ritual bathing (ghusl), repentance, and uttering the two testimonies of faith (shahadatayn) sufficient for someone who abandoned prayer due to severe obsessive whispers (waswas)?

Intention is merely the purpose of an action. The worshiper's knowledge of the prayer they intend to perform is sufficient. If they know what they want to do, they have necessarily intended it. Even if the intention for a specific prayer is absent, but they know they will pray the obligatory prayer of the time, the intention has been achieved. It is advisable not to pay attention to waswasah (whispers/obsessive thoughts) in prayer, ablution, and all other acts of worship. Missed prayers must be made up along with repentance. Neither a ritual bath (ghusl (definition)) nor uttering the Shahadatayn is required.