You can't turn it off

I think one of the most frustrating things for someone with anxiety is having their partner tell them "just don't worry about it." Anxiety isn't something you can turn off and on when you desire. For many people, their anxiety is usually triggered by different things and can be very overwhelming for them.

Do your research

Instead of just not getting it or assuming things, research. It'll be the easiest way to understand them and not cause them extra stress without even knowing it.

Patience

This goes for any relationship obviously but it's super important for dealing with people who have anxiety. Things take time.

Don't try to fix them

Okay, this one bothers me. First off, you aren't their doctor so don't act like it. Secondly, everyone deals with their anxiety in different ways. You can't fix anyone nor are they asking to be fixed. Anxiety is a mental health issue but it doesn't EVER mean that the person is broken. We just handle things in particular ways and react differently.

No comparisons

"This is how so-and-so deals with their anxiety, this might help you." As much as the suggestions may be really sweet, they're not necessary. There are different types of anxieties and different ways to deal with it. Comparing others just because they have anxiety is never to be done.

Anxiety attacks are real

There are many different symptoms of an anxiety attack that many people aren't aware of. Make yourself aware of those. Sometimes when people are having an anxiety attack the best thing to do is be calm. Be there for them but don't cause on any more stress.

Don't minimize their problems

Even if the problem seems small to you, keep it to yourself. Minimizing someone's problems or anxiety is never the answer. Just listen and provide support.