I’m sure that you must have missed your girlfriend and Japan dearly. I guess you have made many wonderful memories there. Well, you can say the following:
Ima Nihon ni sundeireba ii noni.
It would be good to live in Japan right now but….
This expression is incomplete as there is an unsaid portion after the part “noni”. The unsaid portion usually implies strong emotions behind the unsaid portion or a longing that something would occur contrary to the current situation.
So the whole expression actually means “It would be good to live in Japan right now but (I’m not. If I were, I could see you everyday).”
Leaving things unsaid is very common in Japanese communication. Sometimes, it is even more effective that spelling everything out. If you say this to your girlfriend, I’m sure she would understand your feelings for her.
I’m sure you will have a chance to visit Japan again. All the best to you!