How hard to fight the tooth-brushing battle?

My son is almost 19 months old. I've been brushing his teeth/gums since he was like 4 months old. When he was really little he was cooperative enough. As he got a bit older he wanted to do it himself, and I figured it was a good habit, so I let him. I still tried to go over them myself, but more and more he didn't want me to "help" so I let it go. Well, about a month ago we had his first dentist visit. They were nice enough about it, but basically said he wasn't doing a good enough job and I needed to brush for him (and also let him do it himself to build the habit). They also suggested trying a kids' electric toothbrush. I got the electric toothbrush and for a while that was a big hit. I think because it tickled and made noise he thought it was fun, so he'd let me brush his teeth pretty thoroughly. Then I'd give him the regular brush to do by himself. I'd made up a little "brush your teeth" song that we'd sing, and things seemed to be going well. But lately he's just over it. He fights me so hard to even open his mouth. The only way I can get his teeth brushed is to practically pin him down and force him. He cries and fights and I feel like a terrible person. I also feel like this is just making him hate the whole concept of brushing his teeth and that it's going to be worse in the long run.

I just don't know what to do. I've tried all the tricks - letting him brush my teeth, watching me brush, singing, trying to make it a game.....nothing is working anymore.

He does get fluoride drops every night because there's no fluoride added to the water where we live. They must taste good because he loves them. And my mom keeps telling me that she never brushed her kids teeth before we were like 3-4 years old and we all have perfectly healthy teeth. But the dentist was so insistent that it's important, so I just don't know what to do. I can't decide if it's better to ease up and not have him think of tooth-brushing as torture, or just force it every night because it's good for him.

Oh, and I only even try once a day. Both the pediatrician and the dentist said twice a day, but I just laughed in my head. I don't even brush my own teeth twice a day, and there's no way I'm fighting with my toddler every morning AND night!