How does one deal with the obsessive doubt regarding not completely emptying the penis of urine, and the fear of drops coming out after purification (istinja')?

We ask Allah to grant you well-being from the purity-related waswas (whispers/obsessive thoughts) you are suffering from. You must purify the area until you are reasonably certain that the flow of urine has ceased. Then, sprinkle water on the private parts and trousers to cut off the waswas, based on what was narrated by Ahmad, Ibn Qudamah, and An-Nawawi. If urine drops repeatedly emerge, then it is considered salis (urinary incontinence).