Tell him what you've said here.
However, it sounds as though you may have a medical condition, and if that's the case, you may need tests and a diagnosis. You really should have seen a doctor about all this before, so you could present this information to your supervisor earlier. It's not normal to need the bathroom as much as you do, or to take so long to pee.
So, you may not be able to get a note right away. The doctor may want to send you to a specialist. If he does, ask him if he'll write a note saying that you clearly have urinary issues and that he's having you checked out by a specialist. In the meantime, your workplace should be understanding.
And you should probably check with your state's Employment Commission or whatever it's called there on the legality of your workplace's rules.