Hi!
You POOR darling! That must be awful! Those sores are VERY painful. Herpangina is caused by coxsackie virus which is different from Herpes virus and is not an STD. It is the same virus that causes Hand Foot and Mouth disease in little kids. As far as I know there is not oral medicine to make it go away sooner, and will just have to run its course.
Some tips to try: Get a squeeze bottle at the pharmacy (like the ones ladies use after childbirth) and spray water on that area while you urinate, or urinate in the bath tub full of water or shower if you can stand that. Also, I have given my patients Xylocaine 2% topical ointment to apply, and it temporarily numbs the area. Make sure that you take frequent baths and keep that area clean (2-3 times per day).
Also, some people actually get to the point that they CAN'T urinate because of pain or swelling, and if that happens, you will need to talk to your doctor about a catheter--You HAVE to pee somehow!
Good luck!
Hope you feel better soon! (I had that virus in my throat when I was a teen and I still remember how bad it was!)
Dr B