I hurt my best friend beyond forgiveness or apology
Did you have a reason for whatever you did that was so bad?
Yes. He slept with my wife and I slept with his.
You shouldn't have tried to get back at your friend in that manner. Even though what he did was also terrible, you should be the better person and say sorry for sleeping with his wife.
Sex is a natural thing to do. I was on drugs when I slept with my friend's wife and he was drunk when he slept with my wife.
Given the circumstances, I think that what you did is certainly forgivable.  While it is bad to use drugs and alcohol in excess, since it leads to bad decisions like this, the fact is that neither of you were not in a proper state of mind when this happened.  I think you can both accept that you both made a mistake, and should move on from it.