You have answered your own question when you admit that you are yielding to his narcissistic selfishness. You still love him and you want to be back where you were before his infidelity. This will never happen.
It is so hard to break free from a loved one when you still have that love for them, but believe me, staying with him will just prolong the agony, like scratching open a wound.
My mother gave me the best advice I ever had when I was with my child’s father. She told me that I would have a lonely life if I stayed with him. She was right, but I could not see it until years later when I was ready to stop making excuses and enabling his behaviour.
You are strong and you do not need to be co dependent. I understand the loss you feel when you have invested a lot into a relationship, but move on. I hope you posting on here means you are ready to do that.