Are you wearing adult diapers, or are they really the "briefs?" Briefs are like toddler Pull Ups. Actual adult diapers exist, and are used in some hospital settings, but I'd think they would be pretty hard to put on oneself, so I think you use briefs.
Something you might try. It's what my mom does. Buy not only briefs but also incontinence pads. You wear the pad inside the brief, and it will catch the leaks, so you don't need to change the briefs as often. Then you're paying for the pads, too, but you can run the numbers and see how the cost compares. The issue is sensitive skin, and you need to change pads often enough to keep yourself clean. If I were in your shoes, I'd go on the baby aisle at the store and find something there to put on the red sore area, because I think it is for all practical purposes similar to a diaper rash. You're probably already doing this, but I'll just say do use the toilet to relieve yourself, and only let these products catch the leaks. Keeping your skin as clean and dry as possible will help, and if it doesn't, consult a nurse or doctor. Your comfort and well being are worth an awkward conversation. Good luck.
Edit: I see you did say briefs. I apologize for over explaining.
Edit: If you have no issue with the back side, then you could probably skip the disposable briefs and use the pads with regular mens' cotton briefs.