Hi E.~
You are not the worst mom ever! You are a wonderful mom, listening to your child's needs.
This is my understanding: night breastfeeding has been implicated in tooth decay in some cases - mostly the studies that I have seen indicate problems if food sugars are still in the mouth when breastfeeding. So brushing after meals - & particularly at night before bed - is crucial. Additional studies I've seen show that breastmilk can be a cavity preventative, again, when not combined with food sugars. Bottles of formula or milk at night are much worse for teeth (juice too, of course!).
I night weaned my co-sleeping 2.5 yr old for different reasons. If you feel like it will help to night wean, I thought I'd pass along what we found helpful: 1.) I talked to DS about how we would nurse in the morning, & before bed, but no longer at night. He would say "No more nu-nu at night. I know." 2.) think about replacing the night nursing with some other nurturing thing...A song, a back-rub, hugs & kisses. For DS it was stories. We did this for a few weeks, then 'weaned' him off stories to snuggles. 3.) wear heavy sweatshirts & have glasses of water available.
I highly recommend you try to find a dentist who is at least familiar with EBF, & who will gently & slowly interact with your son. I wish I had a recommendation for you - sorry! I would connect with your local La Lech League ASAP (even call the leaders), I've always foudn them amazingly helpful for any breastfeeding related issue. You may also want to post on www.mothering.com dental & breastfeeding forums. Good luck!