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Pray in your mind for help form God as you try to reject any sin. Remember that everything is tiny when God is by your side. Thinking a temptation is "too big" will make it too big.
Second to that, genuinely have no interest in her. This will affect how you act in the outside, they pick up on anything that shows you're interested.
As a concrete example of the above, (try to not think of it if you're talking to her, its more so you understand the second point), some people may reply to things more slowly if they're doubtful of what they should answer. If she detects doubt though, she will hound you more.