What is the way to get rid of severe obsessive thoughts related to the impurity of urine and its splashing, and the impurity of places and used tools, in addition to obsessive thoughts related to apostasy due to the slightest sin or fleeting thought?

All that you are suffering from originates from obsessive whispers (waswas), and its cure lies in turning away from all these whispers. So, whenever Satan whispers to you that urine has splattered on your thigh, do not pay attention to it, and act according to the fundamental principle that nothing has afflicted you. No ruling of impurity is made based merely on doubt or illusion; rather, absolute certainty is required. As for whispers concerning creed (aqidah), they do not harm you and do not expel you from the fold of Islam. You must repel them and not indulge in them.