I respectfully disagree with some of the things others have written. My husband takes ssri's for migraines, not depression. On several different ssri s (Paxil, effexor, too many others to name) when he got on a high dose he complained of feeling emotionally numb. The numbness, in his case, WAS caused by the medication.
My husband is a financially wise man. He carefully considers what he spends. In one case, however, when overdosed on Paxil, it lowered his inhibitions and he bought not one, but two piece of crap vehicles as well as furniture. In one case, I had even forbade him to purchase it, but he had no recollection of it. He became fuzzy brained, couldn't remember names, and fell asleep behind the wheel of the car. He made decisions that were uncharacteristic of him.
No, I don't believe the medication or the PTSD excuses the cheating, but I do believe it (the medications) could have lowered his inhibitions. In my husband's case, when confronted, was repentant. He did whatever was necessary to regain my favor and trust (and believe me, it took A LOT). This included allowing me to attend his doctor appointments.
Cheating and buying excess cars are not the same thing, but there are some behavior similarities here. I believe, if he is not willing to take responsibility for his actions, if he is not willing to re-win your love and trust, if he is not willing to have you work with him to formulate and carry out a plan for the PTSD.....then you should find the nearest curb and set him on it. Or, even if he is willing to do all this and YOU aren't willing to go it with him....that's okay. He cheated on you. You don't owe him anything.