That's something my daughter and I both have to say frequently. She and I have both been diagnosed with anxiety disorder. When we have anxiety attacks, the helpful people around us, who genuinely are trying to help us, try to give us all the reasons we don't have to be afraid of whatever's scaring us.
The problem with that is, as the post title says, anxiety isn't rational.
Anxiety disorder is a malfunction in the part of the brain that controls fear, to put it simply. There's a glitch in the system. Sometimes there's an obvious reason for the anxiety; for example, driving into the city of Boston is likely to give me an anxiety attack, especially if I'm going to a part of the city I'm not familiar with. For many people, that would be a slight fear of running into traffic, getting lost, etc., but they would be able to remind themselves that they know how to drive and will be fine. Others might not be afraid at all.
With me, the fear goes into hyperdrive. My heart races, my breath speeds up, and sometimes I feel like I'm being strangled. No matter where I'm going in the city or whether I've been there before, I'm terrified of doing it this time.
That's when someone helpful will often say, "But you've driven on that road before and everything was fine."
Possibly true, but that doesn't matter. Anxiety isn't rational. It's like someone has taken bucket-loader of water and drenched you with it; the anxiety is the water. And it doesn't matter how many times someone says, "You don't have to be wet"; the water isn't going anywhere right away.
Anxiety can, of course, be managed. My daughter and I have learned to recognize the signs that we're about to have an anxiety attack, and we breathe deeply and visualize a positive outcome to the situation to try to fend off the attack. There are also medications that can help. (We aren't able to take them because of the reactions we have.)
If you know someone who has anxiety disorder, try to be supportive of them without trying to talk them out of being afraid. And if you have it, be kind to yourself. It's okay to be afraid sometimes.