Ask questions and do your best to understand what they are going through
What you have read on the internet about anxiety, I assure you that you do not know the half of it. Try to talk to your partner about it. Ask questions, probe gently, and do not expect to know everything in one go. It may take some time to obtain some information about it. Some questions you can ask are if you can do anything to help when they are experiencing anxiety or if there are some things they do not want you to do.