Aaaaah. Now, with that one simple fact (you’re a girl), the picture snaps into place. Not that I have an answer, but I definitely understand the question.
j/k
You’re doing it already by asking this question. You are doing what you need to do which is to constantly remind yourself that this relationship is never going beyond a friendship. That’s one thing.
The next thing is a bit more difficult. It’s kind of personal. Whenever you have a crush on someone who is unavailable, you probably should ask yourself why this is the case? There could be many explanations. It could be that you are kind of afraid of relationships—maybe because you’ve been burned in the past or because of issues with your parents as you were growing up. Maybe your image of relationships is that they are always disasters. Who knows? Well you do, but I’m not you.
Becoming attached to someone who is unavailable gives you a ready-made excuse for not being in a real relationship. You love someone who is unattainable. Because you love them, you are freed from the task of finding someone who is available. You are stuck on your crush, and that’s that.
It’s just a stab in the dark, but perhaps this resonates a bit?