Dealing with anxiety

Since I was a little kid, I have sometimes felt this inexplicable, vague sense of dread or unease. A kind of hyper vigilance with no focus or target. Just a big red light going off in my body: "I am not safe right now."

This still happens, often without a clear cause. Recently, it's returned. I'm wondering what the reasons might be, and if there are any techniques I can use to manage it, I'd appreciate any tips.