Portrait photography comes with a tremendous sense of anxious pressure that most other forms of photography do not. The nature of working with a living, breathing human that brings with them strength, weaknesses, and expectations can be enough to trouble many new portrait photographer. A shoot filled with anxiety almost never works out. In fact, the anxiety itself is far more harmful than anything that a photographer could possibly be anxious about.
Build Confidence in Everything Else
You may be anxious about working with a model but you had better not be anxious about the rest of the shoot. Show up confidently knowing that you know the photography side of the formula well. In an anxious situation being able to fall back on aspects of the shoot that you are confident in, such as lighting, camera operation, composition, etc. can be a boon to limiting the impact of the anxiety. Invest whatever time it takes before the shoot to be sure that you won't run into technical road blocks that threaten to derail the shoot and shatter what confidence you do have.
Shoot in a Confident Location
If you are new to portrait photography don't double down on your anxiety by choosing to shoot in locations that will accelerate the building anxiety. Instead choose locations that help you feel more confident. The most obvious locations to avoid are public areas with frequent foot traffic. If you feel anxious working with a model, you will feel even more anxious with an audience. Choose a location that is private so that you are limiting distractions that can build anxiety and ruin the shoot. Furthermore, choose locations that you are confident in working in. If you don't know how to light a dark alley at midnight effectively, don't shoot in one until you are more comfortable with portraiture. Choose locations that you most confidently can consistently create great images with.
Avoid Pedestaling Your Model
Your model may be one of the most beautiful people you have ever met but don't lift him or her up in your mind so high that they become akin to a mythical creature. Models are humans, just like you. Connect with them on this basis. Make small talk, break the ice. Develop a friendly relationship to draw on for comfort. But certainly, don't risk making the model feel like you are making a sexual advance in any shape or form.
Create a Comforting Environment
Both you and the model will react positively to a comforting environment that helps dispel anxiety. Play music, have food and drink readily available, and avoid an intimidating tools such as giant lenses (200mm f/2.0, for example). As you become more confident as a portrait photographer you can start pushing the envelope but while still struggling with shoot anxiety avoid any situations that could increase that anxiety.
Solve Problems for Your Model
During a shoot an endless number of challenges can pop up for a given model. By helping the model overcome those problems you both build confidence in your working relationship but also create a helpful feeling that can build your own confidence. A really great way to do this is to stock a small bag of items that models frequently need during shoots such as clips, hairspray, hair elastics, etc. You would be surprised how often models forget to bring such items and having them ready at hand can be a lifesaver during a shoot. Also be mindful of how the model reacts to the experience of the shoot, if they don't seem to be liking the music, change it. If they don't seem to like others watching, quietly make up excuses so that significant others, make up artists, stylists etc. vacate the set. By making your model's experience better you can feed of each other's energy to get rid of anxiety.
Portrait photography is hard, especially to a new photographer. Unfortunately, there will always be a curve that must be learned and experienced before many portrait photographers truly become comfortable shoot. Practice makes perfect, keep shooting. Don't let the anxiety wreck your aspirations to be a portrait photographer. The next shoot will always have more anxiety than the shoot after it. Eventually the anxiety will be gone as you build the confidence needed to work with models and learn that you have the skill set needed to rock every portrait session you shoot.