My gf of a bit over a year called me last week and ended it because of the international distance (never actually said “distance” as the reason, but that’s what everything boiled down to). I called her a few days later after I got my thoughts together. I let her know the conclusions I had come to and understood why it wouldn’t work now for BOTH of us. She texted me a very plain message the other day and I replied later. She responded instantly and I have been NC ever since.
Overall it ended on a good and mature note. I have between 6 and 9 months left in another country and I know she could not handle it. Love is not enough, you have to have right timing and conditions as well.
I didn’t talk about getting together when I move back because I didn’t want to put any pressure on her or build resentment. Unfortunately I know we are both going to see/sleep with other people, but I wanted to find a way of letting her know that I still truly love her and want to be together when I get home. I was thinking about sending a letter (due to arrive a month after NC) explaining that I still want to be with her in the future (make it cool/calm but also humble). Then ease my way back into limited contact and rebuild. Is this a good idea? PM me for more detail and background info.

You broke up. Get on with your life. Don't send her a letter saying that you will still be in love because you might not. If you broke up make the most of your time while you are away. Maybe you will meet someone who will completely turn your world around and will never think about her again.

This doesn't mean you will never get back with her. You might meet her again in the future. But the worst you can do now is wait for her and waste your own time. You will be miserable.