Some people are good with bills. Some aren't. Depression plays a part. PTSD and depression go hand-in-hand. Sometimes people who are depressed (PTSD or not) have trouble dealing with things like opening mail and keeping track of bills. It is too much energy to invest, and they just don't have it.

It is not unusual for a carer to have to take charge of the finances. To do this, you would need a joint account to which you both contribute. The carer would then pay all the bills out of that account as they come due.