I can tell you one thing that you very likely can count on. I very much doubt he is cheating on you. He likes you a lot, even loves you is my guess from what you have written. You also are not the problem. In most with this type of PTSD (much like mine it sounds) what he wants the most is to not hurt you. The only way to do that is to not be with you or if he is then to stay as separate as he can while in the same room or place. When I feel like that which is how I am right now I don't want to deal with other people because it is very easy for them to accidentally do or say something that will make them think they did something bad that they can't figure out. When that happens it is super upsetting and activates the PTSD to the top. I don't want to make people think they somehow upset me because of what they did and that it is their fault. It is my fault and I can't fix it so all I can do is to stay away from people.
Right now for me the worst thing is to hear any sudden sound that doesn't have an obvious and instant explanation. Also is anything that sounds even slightly like violence of any kind, even just people talking outside my apartment using bad language. A lot of that was brought back up full amount at the psych ward which was a very violent place. I have had far to much violence in my life before that, both to me and around me including me trying to save lives and failing, even though it wasn't really possible to save them. They at the ward also tried to give me meds that I knew might kill me but the shrink didn't know his meds well enough to understand that. They finally switched me to another and she immediately understood what was going on. She wrote me up with a complete clean bill of health so I could get out of there. I was out the next day but it was way too late by then. I was in there for three weeks (a different town) and then another three weeks at the ward back here. Psychiatric facilities are about the worst place there is to put somebody with PTSD. It is guaranteed to make it worse.
His work is completely different and he just switches to his work mode. It was like when they stuck me in the psych ward, I just switched into military mode. I knew it would be the only way to get out of there in any reasonable amount of time so I turned off everything I could and just made the bed every morning military style, took a shower, kept the room area perfectly clean and did the laundry, ate the food they gave me and followed all their instructions exactly as given (except the meds). As far as they were concerned I was acting perfectly OK. What they did not notice with the exception of the super nice psych nurses is that when they woke me at night for special meds I did not want I would startle so hard I nearly fell out of the bed. They also didn't tell the shrink about the dreams I had where I yelled loud enough to wake up other people in other wards. I normally do not even talk in my sleep. I don't think they like the psychiatrist assigned to me very much but I will not go into that any further.
I really do hope and pray I will get better. I can't count on it but I will go to church in the morning and ask for some help. I will add a prayer for you as well.