I'm in almost the exact same boat as you. I took "general" riding lessons for several years, stopped when my instructor at the time stopped giving lessons to get back into showing as a non-pro, and spring-before-last bought a younger greenie as a project, and got into dressage on my own. It's been tough teaching myself and Scout, but we are both making progress. We're certainly not show worthy (yet... if the opportunity presents itself and we're ready I'd love to give it a try), but I don't think I'm "ruining" him by a long shot.
This forum is an excellent resource - there are some crazy-experienced riders, trainers, even judges who have helped me many a time.
Here are a few websites that have been helpful as well, assuming of course that you haven't already found them... The Art of Classical Riding--Dressage Training for Horse and Rider ::: Sustainable Dressage - - Welcome to my Site about Sustainable Dressage! ::: Members dressage lessons - Classical Dressage Classical Dressage Notebook - Putting Theory Into Practice
I've been watching some of Chris Irwin's videos on State Line Tack's website. He's a bit of a natural horsemanship trainer, but in the least NH way that I've ever seen, if that makes sense. I find most NH big names are lacking, especially when it comes to riding exercises (horses hollow, behind the vertical, asking for "advanced" movements and "collection" fairly soon after the basics are established, riding the head, etc.). Chris Irwin's Riding Concepts and Basics lessons are a lot more dressage-oriented, with the right emphasis on the training scale, solid foundation, correct biomechanics and moving the horse from back to front. I haven't watched his Progress and Collected videos yet, though... He's not a "D" Dressage guy, at least that I'm aware of, but his methods seem to be meshing well with my understanding of, well, everything, ha ha Breed Dynamics | Horse Videos – StateLineTack.com Video Library
If you ever want to compare notes, discuss self-teaching dressage, theory, or anything, feel free to shoot me a PM.