This is not ready to go, by any means, but here are some firmed up thoughts:
H, I am very grateful that you've taken the time to communicate some of your thoughts to me. Thank you.
I can see why you feel that I've lacked commitment and I recognize the truth in what you're saying. I know my lack of commitment has caused you pain over the years and despite that, you always respected yourself and overall, you handled it remarkably well. You are you all the time, even when it's hard and I respect you immensely for that.
I want you to know that I listed my truck in Auto Trader. I'm sorry I didn't try harder to get rid of it sooner and I realize that it may not bring you much, if any, comfort at this late hour. I see now that it was an every day reminder of my betrayal and I hope you can forgive me for not acknowledging that.
I would love to follow my heart. I can't imagine living my life with anyone but you-when I am with you, I am home. You said apparently it is too late for us. It is never, ever too late to create the future. I can see a future with us together. I can also see that we are not on the path to get there. If you think you'd be willing to work with me on beginning a new relationship, one where we can love, laugh, raise our children and grow old together...then you let me know because I am there.
"Happiness is a butterfly, which, when pursued, is always just beyond your grasp, but which, if you will sit down quietly, may alight upon you."