I don't fit the symptoms I've seen of bursitis but I do get some great swellings on my bot, more usually on the RHS than anywhere else. I have the most comfortable saddles I have been able to find but in my experiments so far I have found the different models of shorts make as much difference as anything.
I'm currently using Pearl Izumi Sugar shorts (as recommended by Charlotte), but even with these I just seem to get sore where the chamois ends and meets up with the seam running up the back of the leg.
I think I know what you mean by the grinding feeling. I don't normally suffer with it on day rides but after a couple of weeks of constant cycling thru NZ, some days it felt like my sit bone was trying to drill through the flesh - especially on hilly days where there was a lot of just grinding it out in the saddle.
Today I winced all through my shower and then had the magic idea of applying some TCP to the affected area. We almost needed a new ceiling.
However mine seem fine after a day of two off the bike where upon the swelling disappears and I'm left with a sort of rough, almost scabby feeling. (Sorry if this is too much info!).
I did notice a big difference when the temperature rose and I went from wearing shorts with longs (without a chamois) on top to just wearing shorts, I think my bum is missing the extra layer.
I don't know what the answer is, but you are not alone.