I'll add my experience in the hope it helps others. We should form a meet up, or at least a mailing list. I bet there are a lot of us here.
The Absolute Best Thing You Can Do in this situation: Learn to recognize and manipulate your own emotional states. Take control over what you bring into the room.
On the worst days, your partner will have little no of this ability themselves. Pick up that slack and things get better faster.
Read Covey, Carneige, and Cialdini for starters. They will together persuade you that you're much more susceptible to the emotion of a moment than you thought you were. And, you'll learn to see how others have controlled the emotions you've felt in the past, to drive your behaviors. It feels like magic, like cheat codes, when it clicks. And that click is the beginning of a real empathy for what you're seeing in them.
You'll also learn how to get someone talking effectively about emotions in a way that loosens the hold, and gets the logical part of the brain re-engaged.
Reading and talking are great, but there's so much more. Find a hobby or group that forces your hand to work nonverbal communication skills. You need somewhere to practice reading others' cues and responding with your own.
Work that long enough, and you'll start to see impending anxiety before your partner experiences the brunt of it. You'll also see the anxiety states they weren't telling you about, some of which they didn't recognize as anxiety either.
At that point, you've leveled up your own emotional awareness you can start leading your partner (mostly nonverbally, sometimes in detailed conversation) to raise their own. That inspires confidence. Confidence is what you want, it's the long con (ha), and a few victories in a row will let the both of you reach for new goals: things that seemed impossible months or years ago just aren't anymore. IMHO, that's the best life.