“How do I stop uncontrollable anxiety?”

Isolate yourself. Sit down, take a few deep breaths to relax. Say to yourself, "I will tell myself to relax over and over until it happens."

You will likely need professional help with that. Uncontrollable anxiety, if it really is that, rather than another physiological response that mimics anxiety, occurs when your mind is acting as if you are about to be killed any minute now and the thing to do is to run like hell or freeze so the predator doesn't spot you. It can be controlled more or less with medication, and there are psychological treatments that are effective in changing the perception of danger and dialing back the emotional response, which is worse than useless even when there is a leopard or an active shooter. See your doctor to rule out a physiological cause. Generally, it's best to try the psychological approach first, because the medications have problems of their own.
I hope this is helpful.

See this link for some insight into your question:
https://my.clevelandclinic.org/health/diseases/9536-anxiety-disorders