Tough place to be, I admire your ability to be strong in the face of such a challenge. I think the mixed message was in your head - if you are trying to make a change then it will feel different for you.
You are being supportive of his decision - that counts for a lot if he is noticing and I am sure he does. You were clear about the message - no mixed signals there. Be strong, hang in there. You are probably too young to recall Jonathon Livingston Seagull's book about love meaning you let the person go and if it real love they will return.
I don't mean to place doubts but I don't believe the PTSD stuff about relationships - it is applicable to abuse - affairs are not considered abuse - just poor judgment and hurtful. He had plenty of time to think things through before M you. Poor decision on his part - maybe. But having said that - if one or two people make a poor choice it doesn't mean you can't make it right again.
