Since you are training for a 12-hour race, I assume you know this already, but there might be people reading this who do not. Do not wear underwear under bike shorts! Bike shorts are meant to be worn commando.

Have you considered your saddle, bike fit, and construction of your shorts? All can lead to nasties.

I hope you get relief soon. Your doctor must not be a cyclist. Perhaps you can find a cycling doctor who can help you out.