Having a partner with anxiety can be very overwhelming and stressful - but that does not mean that it is not worth it and it cannot be rewarding.
Be there for them, always.
Whether they need to talk about something or they just need you to come over and sit in silence with them, always be there for them.
Accept that you will not always understand what they're going through.
If you have never personally dealt with the demons of an anxiety disorder, then it's going to be impossible for you to understand. You may want them to explain it to you so that you can understand, but sometimes even they don't understand how they feel.
Ask them what makes them feel better, then help them achieve what they need in order to feel better.
Everyone has different ways of coping and making themselves feel better, so include yourself in their coping mechanisms. Ask them what they do to feel better and be a part of the process. They may not know what makes them feel better, so observe. Observe their habits and observe what makes them happy and content, and work to turn that into a coping mechanism for them.
When they are having a panic attack; sometimes being told to "breathe" and to "calm down" can make it significantly worse.
Just be quiet and wait for it to pass. If it doesn't make them feel worse, rub their back, comfort them from a distance. When they are having a panic attack, holding them and getting too touchy may just make them feel more stuck when they need to feel free of all pressure.
Do not blame yourself.
There are no triggers for anxiety, it comes out of nowhere for no reason. Blaming yourself for your partner's anxiety will just make them feel ashamed for having this mental illness that they can't control.
Respect their decisions.
Their decisions may seem ridiculous to you at times, but there is a good reason why they can't socialize today, so respect their decision to stay home.
Do your own research.
If you care about this person enough, you will do everything you can to try and make their lives easier. So do your own research, it may even help you understand them better. Research is boring and time-consuming, but if you care about them enough, it should be worth it.
Don't get annoyed with them for worrying about your safety and needing you to talk to them all the time.
Their fears may seem irrational, but that vision of you dying in a car accident is very real to them. They need you to talk to them throughout the day so they know you are okay and so that they know they didn't do anything wrong.