Why can't I get her off my mind even though we've only made eye contact for the last two years?

two years ago a girl in my former class started showing obvious signs of having a crush on me. me being a newbie at this had no idea of how to react, so she lost interest after about one week (average life-span of people's interest in me since I'm not the most consistent guy). I know with certainty that this wasn't some kind of mind play since this girl is very sweet and smart. the thing is, part of why i think she was interested was because of extreme coincidences in hobbies, clothing-style etc. sounds like an uninspired love story, but one time our teacher had given us an assignment to finish a book and make a review on it. it turned out we had borrowed the exact same book and received the exact same grade.

after the summer holiday that year we both had to move to another school. i was desperately hoping that this would be the end of it, but nope. I keep seeing her everyday, in school. even though our homerooms are in two separate buildings, I still happen to have frequent classes five meters from her locker. I also recently learned that she knows one of my new friends, which has made me paranoid

remember, I do not know this girl, at all. we've literally exchanged less than ten sentences during the two years we spent in the same class. I still somehow manage to "unwillingly" catch her eye contact every time I see her (didn't tell ya I didn't find her face attractive), even though I'm 100% sure she's not free. I really have come to the point where I wish I got amnesia just to move ahead in life, but just seeing this one person keeps triggering a spiral down to my former self. it's also killing my confidence since I'm constantly worrying that she thinks I'm stalking her

does anyone have any ideas of how to approach this? the main issue I see is that I simply don't have the guts to go up and speak to her, but at the same time I can't help but notice when she's around. I suppose this is more about mental health than about relationships, but anything is appreciated
Why can't I get her off my mind even though we've only made eye contact for the last two years?
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