How can one get rid of obsessive thoughts related to prayer, ablution, and impurity, as well as blasphemous thoughts, purify the soul from sins, and return to Almighty Allah?

The whisperings (waswas) that you experience are not held against you. You must repel them by seeking refuge with Allah from Satan, consistently engaging in dhikr (remembrance of Allah), reciting the Qur'an, maintaining congregational prayers, and frequently seeking forgiveness. Also, recite the reported supplication: "O Allah, I seek refuge in You from associating partners with You knowingly, and I seek Your forgiveness for what I do not know."

It has been narrated from the Prophet, peace be upon him, that shirk-related whisperings are a clear sign of sincere faith, and that whoever experiences them should say: "I believe in Allah," or should seek refuge with Allah and stop (the thoughts).

The remedy for abandoning prayer is to consider the encouragement for it and the warnings against neglecting it, and to consistently perform it in congregation. As for doubts and whisperings concerning purity (tahara) and prayer, their remedy is to disregard them, pay no attention to them, ignore them, and strive against oneself to do so.