Your boyfriend seems to be a very honest, gentle individual with lots of wonderful qualities to him; and I can understand your heartache in losing him.
It's Not All Your Fault
The first thing I must ask you to do is, come to terms with the fact that this is not all your fault. Your letter seems tinged with much blame and guilt and you shouldn't do this to yourself.
Invariably, it takes two to build a relationship and by the same token, two to end it. I would take heart in your boyfriend's honesty: that it is as much his fault as yours. He really ought to have told you when everything seemed reparable rather than allowing his resentment to build up.
Do Everything You Can
What should you do? You must do everything you can, so that when you look back upon this moment, you at heart would have the knowledge that YOU did everything possible.
But this is the crunch: you can only do so much, the rest will have to come from your boyfriend and voluntarily at that.
He May Distrust Your Promises To Change
Your boyfriend must be drained at having expended so much in the relationship; and hurt that it did not quite turn out the way he would have wanted it to. He may distrust your promises to change. Your wise and eloquent close friend did say the right thing in that "You've lost him, but he hasn't even begun to lose you". Give your boyfriend the space to decide for himself.
Give Yourself Time To Recuperate
Princess, give yourself the time and distance to recuperate, for I think you have been through something traumatic as well. One of the worst feelings you can experience is the regret that comes with hurting someone you love. It may, at heart, seem very uncaring to sit back and do nothing, but your boyfriend needs time to himself to decide.
It runs against all the wonderful and loving things you want to do to make up for all the hurt. But, just as much as you have lost your best friend whom you love very much, so has he. You have done all you can for the moment, now's the hard part of leaving him alone to sort out his feelings for you.
You May Fear Losing Him
I suspect that you may be afraid of losing him (rather than him not loving you anymore). Sometimes people are afraid of losing another because they are afraid of losing a part of themselves. Could this be because the two of you may have spent lots of time together? Its something you may want to consider (please read "How do I deal with my fear of losing him? " on this website).
Ask Friends Who Know Him
How long do guys take to change their mind? Well, that depends on the guy. It's something you need to decide for yourself, perhaps with the aid of friends who know him, who could give you that objective advice. When YOU feel he has had enough time and if he has not responded, I urge you to have the courage to speak to him honestly about what the two of you want.
Life Is Too Short for Regret
Princess, as much as we have experienced what you are going through, we cannot feel YOUR heartache. But this bit of advice I have learnt through much tribulation: that you never stop loving those you have loved; that life is far too precious and short to live with regret; and that you must always respond to the positive things in your life.
I hope I have offered you some insight, and I urge you to respect yourself and your boyfriend's wishes and do the right thing.
PS: This is my first shot at being a Panelist. I would like to dedicate my answer to Tikiri. You are a diamond in the rough, an angel amongst us humans. And I'm glad that you're in my life.