So I think a variant of this approach is useful and a variant of this approach is really harmful. If you say "fuck it, she's not important," you'll be conditioning yourself not to care about her or even actively resent her for "making" you anxious. That way lies a lot of badness.

Nevertheless, I do think it's handy to come to terms with the idea that if she decides to break up with you, then it's not the worst thing in the world. It's an admittedly sucky but manageable state of affairs. You will be a finite amount sadder than you were when you were single! And although you have some influence on her decisions, you have no control over them. So think "I have done everything in my control in this situation. Now I will go play video games/exercise/whatever." This is a more detailed, more accurate, healthier variant of "fuck it, she's just a girl."

Fantastic girls are important! But they're not your whole life! But they are also not unimportant! There's a large range in between those two!