@sm30,
I'm going to come straight out with it.
Learn from your mistake. He "was" your best friend, you feel terribly guilty, it's been eating you alive for a year, what ever you did, obviously was a definite "no, no" and for some reason I'm tending to thing BFWB. Not just, friends.
It's your guilt, not his, get over it, learn from it, don't do it again to the next BFF.