Welcome to PC trevortf. Unfortunately you and your fiance have been trapped in the same kind of dysfunction you witnessed growing up. Also, when the parents struggle to get along and accept their own part in whatever goes wrong and get all "self absorbed" in that, the children often suffer, especially when a parent uses the children for a sounding board or expects the children to side with them and be "angry" at the parent that needed their concerns heard.

If your wife has decided to reach out to a therapist that's a start for her where she is admitting she needs "help". I agree with healingme4me in that going through therapy can bring out some confusing emotional challenges where a person can get worse as they are looking at whatever is bothering them that they don't understand. Often this Mr/Mrs Hyde persona is connected to some kind of deep insecurity where a person can be triggered and become very negative towards others. It can definitely be awful when on the receiving end of that anger and resentment. However, somehow this individual learned that when they behave this way they somehow get what they want which is usually "control" or feeling they have the upper hand in the relationship or group. Interestingly enough this is something that all "primates" do so it's not surprising human primates are so susceptible to this behavior too.

It would be very helpful if "you" sought out therapy for yourself so you can learn "why" you go along with these cycles of how things are good for a while and then there is some kind of flare up where you tend to end up in the dog house like you have described. It's important that you learn if you unknowingly accept emotional abuse and learned how to be the codependent without knowing it.