Not a guy but I understand what you mean by cabin fever.

I'll address the looks issue first. How important are looks to you? Sorry if this is a little crass but how is your sex life? A drop in intensity is normal as you settle into a ltr and become more comfortable with each other, but the sex (IMHO) should still be pretty good more often than average, if sex is important to you in a relationship. (I realise that it's not always the case, but for me it's pretty important so I'm making an assumption but disregard if it's not relevant) If you can find her attractive enough to still be having pretty hot sex more often than not, I think you'll be ok.

Otherwise if the above isn't relevant do you still enjoy looking at her? Do you still find that you hold her gaze and share those kind of intimate moments with her?

As for the cabin fever, if you move in with her without taking a few precautions it will probably get worse. If you make an effort to make sure you have time apart. Have plenty of opportunities to get out on your own or out with you friends/community. If you set a regular times with your GF (I'm not going to use the term "date night" lol) then you should also be able to set regular time out with others/by yourself. Make sure there is enough of "you" in your shared apartment, have an input on the decorating or even better, have your own space. I have a games room and my SO has his shed, whatever works for the two of you so you do feel swallowed up or that you are living in someone else's home.

You should probably think about where you want to be 5-10-15 yrs down the track, do you see yourself with your GF? Does it feel exciting/lovely/where you are meant to be or do you get panic attacks. Do you fantasise about being on your own or without her?

Anyway just something that are relevant from my perspective hopefully there is something you can use, if not I wish you the best and I'm sure there will be something from another redditor you can use.