I once had a self help book on cystitis. I know your DGD might be a lot worse but I was not responding to antibiotics and was fed up of suffering so I tried other ways to help myself. I'll try to remember what the book said as I can't find it on an Amazon search.
One teaspoon of bicarb of soda in a glass of water, on the hour every hour for three hours, so three lots of it. (Tastes awful but did the trick for me.)
Following this, one cup of strong coffee as a diuretic. A jug of water too, be careful not to drink too much as too much can be dangerous too.
On going to the toilet, take a small normal water drinking bottle and put some warm water into it, (nothing else). Wee, then after wiping, pour water from bottle over area and blot carefully with gentle toilet paper, do not rub. Do this every time the toilet is needed. Be very careful with cleaning after defecating as e coli is present and can be a cause of cystitis. Be persistent, do this every time the toilet is needed.
Always wear cotton underwear, no tight pants, no tights at all.
Always empty the bladder and do the bottle wash before and after any sexual contact.
This is all I can think of just now but if I do think of anything else the book said, then I will let you know. The powders in the packets did absolutely nothing for me and so I would end up at the docs, given tablets and they did nothing much neither. After suffering for some time when my ex husband and I first got together, then I found this book and it really did help. Well, either that or I just went off the old man!
I do hope this helps, please let me know. 