OP - I know you are not going to like my answer, but having a trainer with eyes on the ground is INVALUABLE to help you with this. Even if you can only do 1 lesson a month or half hour lessons to start, it will be hugely helpful. I’d recommend finding a dressage trainer that has shown successfully at 4th level or above. Watch some lessons. See if you click with trainer. Make sure trainer will get on your horse during lessons if needed. Watching a professional get your horse on the bit is a revelation.
I’ve had numerous hunter/jumper trainers (and low level dressage trainers) who didn’t even scratch the surface of what my Grand Prix level dressage trainer has taught me about forward and collection. Before working with her, I would read and study books, videos, posts and couldn’t seem to make much progress. Having an educated professional walk you through the feel while you are in the saddle and seeing progress made is an awesome feeling. Understanding collection is a lifelong goal for a lot of riders and a worthy pursuit, but it’s certainly not easy for horse and rider. “Progress” is very incremental.
Dressage Mastery videos on YouTube are another resource I have found valuable - here’s a good one to start with:
https://www.youtube.com/watch?v=iht8yvlnmiQ
I wish you the best of luck with your riding goals!