Hey, don't beat yourself up too much, am sure there are many of us that make a mistake with our love lives. I know I have, more than once.

I think give him time. He's got some wounds to lick and I think it will take him a while. And if your relationship has changed, it doesn't have to mean its for the worst. Perhaps you may, given some time, still be good friends. It may even be better later on, as he will let you go as a romantic interest as being in love with you for so long must've been hard for him.
Is there anything you can do for him to show him how much you value him? I know you are the source of the problem in a way, (but you also should remember that you are right to stop the relationship if it just didn't feel right) but I wonder if there is something nice that you can do for him as a gesture of how much you value your friendship. If he really doesn't want to know then there will be little you can do am afraid, but perhaps its worth a shot.

Best of luck and hugs.