Difficult one without actually seeing the goat....
What breed is she and how heavy milking? What is her condition?
Possibly the easiest would be to take her remaining kid off her completely, and for a while keep milking her out twice a day with proper stripping out. You then have an idea how much milk she gives right now. Then you could reduce her concentrates (but only if her condition allows!) and reduce milking, by initially not stripping her out completely, then only easing her out, then reducing to once a day etc etc. Once she has been milked for the last time I would insert (or get the vet to do it) AB tubes directly into her teats/udder.
Mastitis lumps can be present in the udder for a long time, regular massaging with udder mint certainly helps, but it will take time... also highly likely that she will get mastitis again in her next lactation...
Did you get a sample cultured to make sure you were using the right AB's? Pen & Strep does not shift all strains of mastitis, and my vet now gives Noroclav as a matter of routine.