I am a recovering "Nice Guy" and I will always be changing since reading NMMNG That's my 180 and I live it. I also love the work of David DeAngelo.
I do believe you need to come to a place where your self esteem is more important than anything. You are "Painfully shy" because you hide in fear. Your W even said it. You need to step to the edge of "Fear" and really open up and be someone who is in touch with himself.
Your "muteness" is insecurity. Find in yourself what makes you insecure and challenge it. We are all a walking bunch of imperfect people and we ALL have insecurities. Don't avoid yours, face it head on. IF you like to remain quiet in certain sitches, that's ok too as long as YOU accept who you are and don't give a crap about what others think. Be you, ASK for what you want, and do NOT settle for being anybody you don't want to be. However, don't let fear or insecurity dictate your self worth...EVER. Be a man who is confident in himself.
Life is one shot, enjoy it the way YOU want to enjoy it, but live with integrity.