Anxious over anxiety

I have to take a very important test in exactly 35 days, which will determine if I can go abroad for a Master's degree. I've been studying a lot since July of last year for it, and paid the registration fee yesterday. It's a language proficiency test, and I have to take it for a level above the one I'm currently at, so I've had to study about two years ahead of curriculum on my own.

I know I'm pretty much as prepared as I could expect to be. But I always get really, really nervous before important stuff like this, and so much hangs on this one test. The fee is very expensive and I don't have funds to pay for it again; it's sink or swim. And I'm afraid of my anxiety ruining everything. Of having a panic attack during the test and running out of time, forgetting rules and vocabulary, being so immersed in my fears that I don't hear the test audios correctly, mispronouncing words, etc. That stuff has happened before; there have been tests at school and university where I felt super dizzy, could not stop crying and couldn't write or speak properly. I'm afraid of it happening again at the worst time. The possibility of anxiety is making me anxious, if that makes sense.

I know I'm in control of my thoughts up to a point. But I've had anxiety for so long that it sometimes feels like this all-powerful entity that can just show up whenever and take over me, and I'm powerless to stop it. It's not like this at all, but it sure feels like it.

If you tend to see your own anxiety in this way too, how do you handle it? I'm in therapy and taking medication, are there further steps I can take to lessen this "anxiety over anxiety"? What can I tell myself and/or do to take my mind out of this loop? I have a month to work this out and get a grip on myself, I can't let all my effort go to waste.