For example, on a policy level, Japan never has them. They just don't immigrate any of the religious group who might commit terrorist attacks. So it's a non-issue.
And on a personal level, you can mitigate terror attack risks. Just avoid gatherings of people, basically. You can go as far as you want with this - living off-grid in the forest makes you pretty much fully insulated from terrorist attacks.
So the only question remaining on a personal and policy level is what the cost/benefit is for various things you could do to prevent terrorist attacks.
This is a common mistake. People like to think that some options are impossible, or some gains are free. Thinking this way makes decisions easier; it makes you feel more secure in your opinions. In reality, everything is possible, and everything costs.