Getting in front of depression requires communication and honesty. Some folks are terrified of telling their partner that they have depression. There is a high risk factor associated with being vulnerable about this topic, but it can just as easily be compared to having a food allergy. Be honest about it, do your best to not make it someone else's issue, and accept it as part of you. You will find it's so much easier to operate in life when you are able to accept what's in front of you and spend your energy focused on the future.
Walk gingerly. Go back to your values. Love your partner. Offer kindness. Make grilled cheese. Don't assume. Ask questions. Hug. Get a physical. Check meds. Call the therapist. Watch funny movies. Do yoga. Take time and space for yourself. Induce hope, and try not to judge.
Depression sucks. There's no way around that, and for couples trying to keep a healthy relationship together, it can be a force to be reckoned with. But I promise you, it can be reckoned with.