It sucks to be in this position. Encourage her to get therapy and whatever other forms of professional assistance might be available and necessary for her (sometimes hormone issues and chronic pain issues can trigger anxiety and need physical treatment), but be kind, don't push things, and try not to overthink it. I know that's harder to do than it is to say.

It's very hard to strike a balance of empathy and positive, upbeat encouragement without overcompensating into enabling territory. You can be supportive of a person with anxiety without enabling their unhealthy coping mechanisms. If she cuts, drinks, smokes, engrosses herself into something that actively makes her anxiety worse, or just wallows in it, don't be afraid to let her know about your observations (although it's best to phrase them in a manner that is concerned but accepting, not accusatory or panicky).

Aside from that...being with someone with anxiety is basically learning how to handle being alone 99% of the time. You'll have to learn to keep your problems to yourself, sometimes for hours, sometimes days at a time. Sometimes you have to just internalize stuff or find friends you can vent to, and hobbies to get out your emotional energy. Listen actively and don't offer advice unless she asks for it or seems genuinely frustrated to the point of voicing that she doesn't know what to do. Try not to compare what she's feeling to things you've felt. Don't worry about trying to explain your feelings.

The sad truth of it is that while people with anxiety do care about their loved ones, the pressures of that anxiety tend to come first. You might be having a terrible day, but if she's having a panic attack you have to pretend you aren't. If you're both sick, and she's falling to pieces, you might have to suck it up and take care of her while neglecting yourself. If you're fighting and it's clear she just won't change, you might have to learn to adapt. Having a mentally ill partner requires a lot of adapting.

There's no shame in it if you decide you can't deal with a relationship that involves an unequal partnership such as the one between a neurotypical person and someone with severe anxiety/depression. Be honest with yourself and consider what you want and need before you make her any promises. Remember, you're no better of a person for staying with someone and being unhappy than you are by splitting up over an honest problem and working towards your own happiness. You owe nobody anything.