WOW firstly i have to say you have done soo well to have a 6 m/o who has not yet been sick T had a cold at around 6 weeks which i dont believe was any fault of mine and it was just awfull!
as tigergirl said there is no need to feel guilty these things will happen and it is definately NOT your fault i can only sugest that perhaps you buy some infant pentavite (liquid multivitamin) they have one which can be taken from just 3 months of course this wont make her better neither will it stop her from getting sick but it just may give her a bit of extra immune support and perhaps help her get better sooner!
also do you breast fed her ? i think it helps a bit if you include lots of garlic in your diet as it is a natural antibiotic and some of it will be passed through your milk
best of luck i hope this passes quickly for you and her take care