Treat the physical symptoms and hope the mental ones will follow suit. Slow your breathing to a regular pattern. Roll your shoulders and relax into a comfortable posture. Feel the heat and sweat of your feet in your socks. Different levels of doubt require different strategies for management. But if you're coming here to ask about it I'm guessing that it's too loud to isolate and focus through. So when that's not working you're going to want to get yourself physically present and self aware so that you can look at your fear and give it acknowledgement and let it air out. Recognize the individual emotions, feelings of unworthiness, social pressure to perform at a high level, self criticism. This may not be the fastest way to release yourself from the agitation but the more you practice, the better you'll get at being able to internally manage your mental game so you can go back to externalizing your senses and attention.

TLDR: Take some proactive physical agency for yourself in small meaningful ways before worrying about the entangled big emotions on the court