What should I do about the obsessive-compulsive disorder (OCD) regarding purity that makes me feel that water splashing from the bathroom floor is impure, and prevents me from entering the lavatory?

The cure for obsessive thoughts is to disregard them and not to rule anything as impure except with firm certainty, a certainty upon which you can swear an oath. The floor of the bathroom is pure, and whatever comes into contact with it is pure. No matter how much Satan whispers to you to wash something because you doubt its purity, do not pay attention to this obsession; rather, turn away from it.