I agree...you need to work on the bad side more than the good side right now. And in small increments; meaning ask for a little bit of bend, release, ask for a little more bend, release, etc. Reward the slightest try at first, and just gradually ask for more and more. You'll get there, just keep at it.
I went through that same sort of thing with my mare because she had so much tense muscle build up in her neck from being high headed and nervous for so long before I got her; so I had to start flexing very gradually...asked for a tiny bit of bend, and eventually she was able to bend all the way back to my foot...but it took a LONG time to retrain those muscles in her neck, and loosen them enough so she actually could bend.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."