Hi,
I am so sorry to hear about your problem! Welcome to the forums, by the way.
My second child also had some problems with tooth decay, ands the truth is that we will never know the exact cause of the decay. My first child's teeth were absolutely perfect, and their eating habits and other life conditions were identical. It is a mystery.
I breastfed my child with tooth decay for two years, and I did not wean when the dentist advised me to (he also blamed being a vegetarian!). I did do my own research and concluded that breastfeeding itself is not at all harmful for teeth. What can be a problem, though, is milk pooling in the mouth throughout the night, when the teeth are not completely clean.
Food particles and breast milk can work together to provide food to bacteria that cause caries. If you are going to continue night feedings, I suggest making sure that your baby's teeth are absolutely clean and free of food particles when you go to bed.
You could also consider night weaning.
There are all kinds of different schools of thought on the topic of early childhood caries, and even more so when you add breastfeeding in the mix. At the end of the day, you have to make your own decisions. I wish you all the best with that.
Oh by the way, my kid's teeth were fixed at the dentist with only local anesthesia and they are beautiful today. It will happen for you, too! Hugs.
Olivia