No, you're not. It sounds as though you're already hurting a lot.
Caring for someone with dementia when a marriage is already strained is very difficult - impossible for some. You've only got to look around the forum to see how it has broken relationships that used to be good, never mind those where things were already difficult.
You don't have to do this. You do have choices though none are easy. You are right in that things will get worse, possibly a whole lot worse so it's a good thing you are reflecting on your future. Not all of us are cut out for caring (I am definitely not!) and not all families are happy families.
If you really can't do this, you could leave the marriage. You can also contact social services and tell them you have reached carer breakdown and simply can't do this anymore. Tell them you can no longer care for your wife and they must arrange care for her (following a care needs assessment). They have Duty of Care, not you, but you may have to remove yourself, perhaps temporarily, before they will take notice.
Don't let your wife's dementia destroy you too.