I'm sorry for your pain. I know how much it hurts to know that there is someone else.
There are a lot of ways to handle this, but the only way that is the right way is the one that benefits you. Right now you're in pain, but you are angry too. Any decision made from anger is not a good one.
Any ultimatum or decision or choice you force upon him right now will only justify what he is "thinking" already. I would go very dim and focus on you til YOU can make a rational decision.
This is where the support of the board and the alt will help you. Can you hold back from lighting him up or giving him an ultimatum for at least a few days?
