Wow! Do I know that feeling! My oldest, at age 7, has had 2 extractions and 4 crowns, plus multiple cavities filled. Did I feel horrible whe we went in for one of our first couple of visits to learn that he needed root canals? Root canals at not even 3 years old? You bet I felt like the worst mom on the planet. Even after the dentist said there was not a trace of plaque anywhere, no bottle rot, and no cavities on his front teeth (that he's had the longest). Even after he said that cavities often start between the teeth and by the time you can see them, its almost always too late. Even after he said that he could tell that we brushed his teeth, and flossed, and took good care of him...still worst.mom.ever. Years later, we continue to send our dentist on vacation:) Like a lot of other posters, my youngest (5yo) insists on brushing his own teeth, and has for years. Of course, I make sure that he is doing a good job, but still, he's five. I think he's had one tiny cavity that was filled without needing any numbing gel or novacaine. His teeth are gorgeous, and white, and tough as nails.
Don't be so hard on yourself, you sound like a great mom! And the fact that this bothers you is testament to that. You can only control what you can control. Roll with the punches and know that teeth are not one of those things. You can do your best and that's enough! Do your best to keep them in his mouth, so that his adult teeth have a space to grow into. That's our goal...