This is what mostly works for me:
0) Ensure you're truly in love with your SO, and vice versa. I'm going to assume this evaluates to TRUE for what follows.
1) Have your SO read PG's essay on this topic; you know the one.
2) If their response is receptive and nurturing, then talk about concrete tactics that will support the long, uninterrupted stretches of time you need to thrive [tactics to follow, below].
If they get defensive after reading the essay, then you will know that indulging in maker time when you could otherwise be spending time with them will need to happen A) in a separate, unshared space, and B) during hours and within contexts that the general population of non-makers would consider a socially acceptable time to be practicing your craft--i.e. 9-5, M-F.
If (ideally) your SO inquires as to how they can support your maker's schedule while you're cohabitating, then explain to them how the Pomodoro technique works, and on your breaks between Pomodoro's, give them your complete and undivided attention. This way, you're able to focus without stressing about possible interuptions, and your SO enjoys your full, undivided attention during breaks, with the nice side effect of you being in a great mood during these breaks.
3) Realize that your SO, and all the other relationships you have with friends and family, are at least as meaningful and valuable as anything you could be making. Share the joy and purpose that making brings to your life with the people who love you--and just as importantly--take pride and great pleasure in the things that matter to them, too!