The next time you're overrun with anxiety about something that could go terribly wrong, don't try to reassure yourself that everything is going to be ok.
First of all, it's not going to make you feel better. Positive visualization doesn't work all that well. If it did, we would all just positively visualize away our fear of public speaking.
Secondly, everything may not actually be ok. If your anxiety is the result of a speech you'll soon be giving to a large audience, a lot could go wrong. Your slide deck may not work. You could misspeak. You could forget words. You could trip and fall into the audience.
When you try to reassure yourself that everything is going to be ok, you're lying to yourself. You can't possibly know everything is going to be ok, and deep down, you know that. Hence the anxiety you can't quite shake.
Instead of trying to visualize the best case scenario, or convince yourself that everything is going to be ok, consider leaning into the uncertainty by asking, "What's the worst that could happen?" Then be honest with yourself. What is the worst that could actually happen?
If you're terrified about a public speech you'll soon be giving, consider the worst case scenario and think about it in very vivid detail. You could forget your words and be left standing in front of the audience, frozen both in mind and body, sweating profusely. Then consider that possibility all the way through. What would happen then? You may, in utter embarrassment, run off the stage. Some people (read: assholes) may actually have a few laughs at your expense. Someone may catch it on video and share it on Facebook. Maybe the video goes viral and you're branded with the terrible experience for a long time. It wouldn't feel great, but life would go on, just as it has for others in similar situations. Some time far, far in the future, the video on Facebook may actually make you laugh.
This approach of embracing negative visualization dates back thousands of years. It was referred to as "the premeditation of evils" by Stoics. It works because the specificity of the visualization strips the future of its uncertainty, and uncertainty fuels anxiety. It also works because, unlike positive visualization, it doesn't require the future event you're worried about to go ok for everything to actually be ok.
With practice, this technique works incredibly well for any event in your life causing you angst, large and small. Nervous about not making a deadline on a project? Afraid of losing your job? Waiting on cancer test results of a loved one? Visualizing the worst case scenario prepares you and your mind for what could actually happen, which is particularly important if any part of your fear could actually happen. Positive visualization does no such good, meaning when something bad actually happens, you are left emotionally and intellectually unprepared, which makes matters worse.
So ask yourself, "What's the worst that could happen?"
Some additional, more in-depth resources on this to help get your started:
The Antidote, by Oliver Burkeman
The Power of Negative Thinking, by Oliver Burkeman
The Obstacle is the Way, by Ryan Holiday
The Surprising Value of Negative Thinking, by Ryan Holiday
The Practicality of Pessimism, by Tim Ferriss