My son who is now 28 months old was only 13 months when we found out he had 6 cavities. :( We were also told to night wean. After viewing some resources, getting information and failed attempts at night-weaning, I decided that breastmilk was not the issue. (although I had read that food mixed with breastmilk is bad, so I would brush after each meal or before he nursed) I would brush his teeth before bed, and put flouride on his teeth (dentist's suggestion). Then nurse him through the night as usual. I also used Dental Essentials (www.dentalessentials.com if you want to look it up)
It seemed that his teeth only got worse. Nothing was helping. By the time he was 22 months, his top 4 teeth were severely decayed. 2 of them weren't even teeth, they were decayed up into his gums. Horrible I know! We just kept getting told they would be able to fix them when he got older without GA (general anestesia). We were very misinformed.. I ended up weaning my boy at 21.5 months (he was only nursing at night) figuring that night weaning was the only thing we didn't try.. :( It was a sad time for me. I loved our nursing relationship.
Finally we were referred to a pediatric dentist who was amazing. Although, because his teeth were so badly decayed, and soo close to infection, we had to do GA as soon as possible so that his adult teeth weren't affected. It was horrible. His top 4 teeth had to be pulled, they were so far gone, they couldnt even cap them. And his top 1 yr molars capped. The GA experience alone is enough for another thread.
Anyways, I say all this to say, that I wish I would have had all this knowledge that I have now. There are so many natural ways to prevent and even reverse tooth decay. (cell salts, xylitol, etc. Let me know if you want more info) I could have continued breast feeding had I known all this. Another thing that I found out, is that sugar that you eat, goes through your breastmilk. Just like alcohol would. Thats the only example I can think of. I ate a lot of sugary things. I had a definite sweet tooth. I would eat a sugary bowl of cereal every night before bed, and that was the breast milk that my son drank through the night. Wish I would have known that. I agree with PP's that say genetics play a part. Some people have strong enough saliva to kill the bacteria in their mouths without help, and some people have to do lots of things to prevent it.
So to end, I think you can definetly continue to breastfeed! Just need to read the right information and resources to get the knowledge to prevent any further damage to your daughter's teeth. There sure is a lot of information out there, especially on MDC. In the dental forum.
Your not alone, and good luck with everything! God Bless :)