I agree with your friends who have advised you to “skip the race”. You are only 27 years old, your training will not have gone to waste, and there will be plenty more races in your future. You should definitely take time off, work at being well hydrated and get plenty of rest.
The following is more important. While you probably have either bronchitis or asthma, either of which could be precipitated by a sinus infection, we should not assume that that is the correct diagnosis. Getting a severe bout of infectious bronchitis in the summer, resulting in “extreme tightness” of your chest and, “trouble taking deep breaths”, would be relatively unusual. And this is especially true if you do not smoke cigarettes and I would be very surprised to find out that an athlete, such as yourself, smokes.
Most important is that you and your doctor consider the possibility that this is not simply sinusitis with secondary bronchitis. Two diagnoses to be considered are: 1) pneumonia, despite the allegedly clear chest X-ray and, 2) blood clots to the lungs (a likely possibility if you are taking birth control pills), either of which could cause your symptoms, especially the extreme chest tightness, and would be especially worrisome if the “extreme tightness” was experienced by you, at a time when the examiner heard good breath sounds with little or no wheezing.
I suggest that, unless your symptoms have dramatically improved by the time you receive this message, you arrange to be re-examined and re-evaluated with consideration of pneumonia, blood clots or some type of non-infectious, allergic pneumonia.
Good luck