I agree with others that a doctor's appointment is in order. He/she'll almost certainly order a urine culture for a UTI, but that's problem worthless. Unless you've had a UTI your entire life, that doesn't make sense a cause. You have to have done physiological, or anatomical anomaly at the root for the problem to persist 18 years.
The doctor will recommend tests, and a course of treatment. Sometimes the verb used is "order" rather than "recommend", but remember, the doctor works for you, and you can refuse any test of treatment. If you want to try desmopressin, ask about it. The Dr. may want to rule some other things out first, but that's not really an unreasonable request.