The PhD will end at some point soon, however, your partner will always be there...if you're meant to be together. I say complete your PhD and then go back to your country. It's tough now to transition from being together to being alone in the US, but remember, it will come to an end, no situation is permanent. Also, in my thinking, if your situation was reversed and you were done with your post-doc, would you be okay with letting your SO quit their PhD program just because you finished yours ahead and want to move back home? Don't quit, you just have to work harder at your relationship in the time you'll be apart.