There will always come a time when students-- if they are diligent, if they learn from you at all-- will not need you to hold their hands and walk them through life's (or in her case English's) rough patches. When you encounter anyone as bright as her, as apt to pick up what you know... well, a part of you doesn't want it to end. Even if you know it must and that you have to let her go when she can learn nothing else from you.
What's nice is that long long after you've done with mentoring someone who resonates with you, he'll still fondly refer to you as "Teacher" or "Sir." Even if, by now, you're nothing of the kind to this person. Happily you can both progress as "friends" if the teacher-student bond is still strong long after it's become obsolete.
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I had wanted to say more here, but I'll keep this short and easy to read. There are always other posts.