I'm currently reading Thich Nhat Hanh's Peace Is Every Step [0]. In it he says something similar. It's not that worrying is not useful. You need to worry for sustenance, shelter. But a lot of thinking that we as modern humans do is useless. Me going over over on why I didn't get that job, or why did my colleague say or ..., you get the idea. Mindfulness can help with this a bit where if you find yourself caught in a useless thought loop, you can step back and try to figure out if you do have agency, if not, just move on.

[0] https://www.amazon.com/Peace-Every-Step-Mindfulness-Everyday...

> It often makes me wish for a non-religious religion -- just a refuge of tranquility.

Have you looked into Buddhism yet? Because that's exactly one of the ways I'd describe it--a non-religion whose focus is on tranquility.

If you haven't looked into yet, this is a great easy-to-read book to start with: http://www.amazon.com/Peace-Every-Step-Mindfulness-Everyday/...