His job is not just "uterus". I'm surprised he would say such a thing, that there's nothing else he could help you with. Wow.

Wish I could help. To me, it sounds like a bladder or urethral infection, or injury from a previous infection. However, I am not a doctor, and obviously you have been tested for that. The symptoms just seem familiar. You've never had blood in your urine (even microscopic hematuria or hemoglobinuria), or lower back pain, nausea or fever for seemingly no reason? Never passed a kidney stone?

The only other thing I could suggest is kegel strengthening, and bladder training ie. practicing holding it for at least an hour, if it's possible, and seeing if you can lengthen that interval over time. When you get used to urinating so frequently the urge comes more often, and it's possible to desensitize yourself a bit with practice.

Good luck, this doesn't sound comfortable in the least!