Literally, right now, breathe in deeply to the count of four and out to the count of four. Extend it to 5 if you can. Then keep in breaths to count of four or five and let out breaths be as long and slow as you can gently make them. This calms the nervous system. While you do this breathing say in your mind: 'Right here, right now, I am fine. All I need to do is breathe. Nothing else. All is well right in this minute. I am breathing.'
This reassures you that you don't have to do anything to justify your existence. All you need to do is breathe. Gradually you can train yourself to believe this in a Zen-like way. All we actually need to do to be alive in this world in a given moment is to breathe. Everything else is choice. Basic food, water, shelter, companionship etc matter too of course but in any given moment, breath is all we need.
Longer term, journalling is good. Write down an anxiety that plays on your mind, and then write down as many other ways to think about it as you can.Imagine how a super confident person would think about it, or a relaxed person, or a problem solver or a Pollyanna type. Look at the benefits and drawbacks of being anxious about this thing and decide what the most powerful and effective way to think about it would be. Even if you don't actually think that way right now, write down a statement as if you did.
Eg. Suppose you dread going into work and it makes you sick and terrified. You might end up saying: 'Work is just part of my life. I choose to work to pay my bills. Work provides me with that stability so for now, I choose to go in. Even if I got the sack, I'd find another job, People do, every day. So I don't need to worry about how my performance is rated. I don't need to like the work or my boss and I don't need to stay forever. I can and will look for more suitable work but for now, it doesn't kill me. I stand up for myself against bullies, I keep out of office politics, I do what I can as well as I can every day and I take pride in that. It's just a job. In thousands of years' time, the world won't turn on a different axis just because of how I do my job. It is not that important in the grand scheme of things, so I won't let it rule my thoughts. Right now I choose to think of something else anyway. I enjoyed that new TV series/bookI was reading/music on Spotify. I will think about that instead.'