I take most of my WIW photos in the mirror because it's easiest for me to do quickly. I used to think the same thing though, that I always looked better in the mirror than the camera, even between pictures I took in the mirror.
What I found that as I took more and more pictures of myself and had more pictures taken of me, I got more comfortable looking at myself in pictures and the two versions just kind of came together in my head where I don't think that now.
I agree that both cameras and mirrors can distort, but at least for me, the biggest difference was that I never took or had a lot of pictures taken of myself before, and I'm much more comfortable with it now.
A similar brain phenomenon that has always happened with me in pictures is that I would hate them when I first looked at them, but a couple of years later I would think they were good pictures of me.
My advice is to take more pictures, and post them too. Sooner or later you won't bat an eye. I have even posted some middle of the night crazy hair pictures where I was trying something on recently, and while I can cop to the fact that they weren't flattering pictures at all, they were for a good cause and didn't even come close to ruining my day as they might have done in the past.