Sorry to hear that things aren't going so well between you both.
First, I'd like you to know that INFJs are (unfortunately) known for being the least satisfied type in marriage.
If you visit this sub often, you'd see that we have a tendency to visualize the "perfect" outcome of a situation. When that situation does not live up to how we envisioned it, we have a hard time adapting to what is actually there.
Your girlfriend is probably going through something like this now: She envisioned the perfect living situation, reality hasn't aligned with her vision, and she now may be questioning whether this was a good idea.
Another similar theme amongst INFJs is that we are constantly in search for something better than what we have. We idealize everything: Our careers, our partners, our destiny, etc. Again, this makes it very difficult to appreciate reality.
Our idealizations of the outside are mere reflections the idealizations that we have about ourselves. Most of us are on a constant journey of self-improvement and have a tendency to believe that improving our external environment will somehow make us feel more fulfilled internally. In most cases, this couldn't be further from the truth.
With those ideas in mind, just know that your biggest hurdle will be combating your fiance's visions. Her inability to completely accept and appreciate who you are as a person, the current living situation, and whatever else will ultimately cause her to retreat and rethink things.
My advice: Do not act like something isn't wrong. She needs to talk through these feelings, whether it be with you or a good friend. She may be holding back her real thoughts/feelings so they don't hurt or offend you, so you need to show her that you can handle the truth. Maybe take her out on a date and casually bring it up: "I've noticed you've been really quiet recently and I want to help. Is there anything I can do to make you more comfortable in our home?"
She needs to talk about what's going on, or else her thoughts will continue to fuel her unhappiness and potentially cause her to make irrational decisions.
Hope this helped. Feel free to PM me.