People who have anxiety are very troubled souls. They are constantly with themselves and their surroundings and what they go through can sometimes totally be out of hands. They will freak out at the smallest of things and they will struggle with that for hours. Their anxiety will get triggered and then they will spend days battling with it because it keeps recurring. As someone who goes through anxiety attacks very often, I know how one needs to be handled and dealt with at that very moment. Here is what you should do:
You stay calm.
The first thing you need to do while your partner is having an anxiety attack is stay calm yourself. It is not easy to see your partner bothered and disturbed, and you really want to help them. You are probably worried for them but if you panic and freak out yourself, it will only make things worse for them. Stay calm yourself and then try to help them.