I've been following and hesitated a while before commenting. I'll be blunt but not cruel.
Oversimplified - you can't. Because you are the reason for the darkness - he knows that and that is what hurts him the most.
BUT (hopefully you are still reading)
What you CAN do is a bunch of the following:
* get tested for STDs and do all the follow ups the Dr recommends
* Read from the Healing Library [top left]
* Read all the recommended books from those who commented on your other threads. There is wisdom in them. [also podcasts, etc.]
* Be authentic. Come clean about EVERYTHING [trickle truth kills any healing that may have been done]. Not berating you and if you have no other secrets good for you - but since many still do hold a few things back, please don't be one of them.
* Accept that he is on a Roller Coaster ride from HELL. He is angry, sad, numb, rage filled, sobbing, hysterical, doesn't even know his own name, barely functioning, hyper functioning, mindless, hyper focused, etc. all at the same freaking time!
* Stay calm. He will hurl some zingers at you. Stay calm. Do not reply in anger, blame him, make excuses etc. In other words - stay calm and own your shit.
* Don't love bomb him. Please. We BS can see through this and it really is an insult to injury.
* Gently, imagine your life without him. What would that look like and entail? Sadly - since it may come to that - you should be prepared.
* Keep a diary, journal, etc. Your feelings need to be expressed also. He, however, shouldn't be your sounding board for many things.
* Get an IC - keeping with the "he's not your sounding board theme" you may need to purge things from your system too. An IC is a good place to start
* Ground yourself - be it spiritually or physically. Attend services if that is your thing. Attend yoga if that is your thing. Go on walks and nature hikes if that is your thing. Don't know what your thing is - find out.
* Don't neglect yourself. You still need to stay hydrated, nourished, rested, hygienic.
* Be there for him
* Give him space when he needs it
* Talk when he wants to
* Talk when he wants you to
* keep posting. SI has gotten me through my darkest days. And I have gotten sage words of wisdom from both BS and WS alike [even if I wasn't ready to hear what was being said, later on it was remembered and appreciated]
You have a long uphill battle. I don't know the outcome. I do know you have the potential to become a better person on the other side - whatever it looks like.