When I went on my first interview I was lucky enough to be young and happy and anxiety-free. Since then, my circumstances have changed and there are a few things I've found really helpful (I work in HR).
1. This IS stupid and prejudicial: Walk into every interview as though you are a positive acquaintance of the interviewer. Beyond your qualifications, they want to know that you are prepared and you can get along with them. In short, they want to see if you're someone they can bear to work with every day. You can be completely 100% perfect for a job but if the person after you is slightly less qualified but hits it off with the interviewer, they're more likely to get hired. Be mindful of your manners, smile, and if you can find a way to connect with your interviewer it will make all the difference. Don't get too comfortable, but walk into the interview as though you've met this person before and it's just another positive exchange.
2. Bring a cheat sheet! Take a look at your resume and be prepared for questions about gaps of unemployment, why you would be a good fit, how previous experiences apply, accomplishments--anything you need and want to be known at the time of your interview--write it all down and bring it with you in a folder or on a clipboard for easy reference. Also, list any questions about the company you may have and ask them! You'll feel more confident, and it gives the impression that you are serious about the job and you've done your homework. This is important because it lets your interviewer know that you've prepared for the interview and take it seriously--they do not want to catch the blame for a "bad hire" (ugh).
3. Know what hiring managers are trained to look for---gaps in employment, unreliable references, non-transferrable skills. Google HR Employment guides for the industry you're looking to work in and see if you can find helpful information. Think of what you would want if you were hiring for that position and be prepared to explain why certain concerns will not be an issue for them. A resume is great, but I want to have a conversation with the person I'm looking at and look into their eyes and see sincerity. Reach out to your references and let them know you're applying for jobs and you appreciate them serving as a reference for you--a gift certificate for $5-10 to Starbucks after you've gained employment will express your thanks and motivate them to continue to speak positively about you in the future.
4. When you successfully complete an interview, reward yourself whether you get the job or not. You showed up and did what you could and that is an amazing step forward. Keep in mind that you are not in complete control of how others see you and your anxiety is not to blame for not getting a job but you can control how you move forward afterward. You took a step forward and if you continue to make those little steps it will land you in a better situation. Learn from bad interviews and keep going--idk about you but I was never taught "how to interview". I remember that during times I've been the interviewer and interviewee, but not everyone will. They don't matter. Learn from your experience and keep it moving. Shut down bad thoughts and spiraling into your anxiety. Don't let yourself fall into harmful self-talk. "I didn't get the job bc I'm not good enough, I'll never be good enough, etc."
4. Beta-blockers/anti-anxiety meds and supplements. Just be patient with yourself until you find the right balance of natural/psych prescribed meds* that work for you. The most amazing psychiatrist I ever had introduced me to a combo of vitamins during our first session. I was reluctant, but I realized after taking them that my eating habits were a major cause of my physical symptoms and while the vitamins weren't a cure, it certainly helped and I still turn to them before anything else when I notice my anxiety/depression is spiking. I am NOT A DOCTOR and before you take anything recommended to you by a rando on the internet you should absolutely do your research and consult a medical professional as well as look for possible interactions with any meds you may be taking. She told me to take a multivitamin, omega complex, b vitamin complex, calcium with vitamin D, fish oil and magnesium. My depression left me unable to leave my house which meant I wasn't taking care of myself, getting sun, hydrating, eating clean--all this really took a toll on my mental health. I can't tell you how much that alone changed my entire life.
*BE CAREFUL WITH BENZOS: if you weren't aware of the massive interactions/capacity for addiction/torture of withdrawal visit madinamerica.com (not my site, but is a very informative resource about the overmedication and lack of education in the psych field)
I hope this helps. Good luck on your job search and if possible, exhaust remedies with the least amount of abuse before turning to meds like xanax or klonopin for anxiety.