Practice!
If you find it impossible to ignore the score and focus on execution, than practice that. For example, during practice, at any point in a game say to yourself: "ok, I have the front 9 strikes. I must throw the next 3 for the perfect game". Imagine the scenario is real. Fix it in your mind. Try to throw the next three strikes as if it is the real thing. The more you do something, the better you'll get at it.
If you can ignore the score and focus on execution, that's the best road to take. With it must come the realization that sometimes you execute well and the results are not pefect. Other times you do not execute well and the results are perfect. That's the game of bowling. All you can do is execute at your top level. Once the ball leaves your hand, it is out of your control. If you execute well enough, long enough, the scores will come.
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Kill the back row (or maybe this should read "make your spares, dummy")