30 seconds and the foundations of my progress were shaken

This seems a little melodramatic next to some of the more intense subjects here, but I need to vent.

As I got to know her, I developed a crush. No big deal, I've had lots of crushes before. Suddenly she's all I think about. As an awkward teenager with low self-esteem I cut down my contact with her, hoping those feelings would go away and I wouldn't have to embarrass myself. The feelings only intensify. Wait, that didn't happen with the others. For approximately two years I thought about her for a good 20% of my day, every single day. Every, every decision I made, every path I took was influenced by "how will this make me look to her? What would she think of this?" and so on.

Then I found someone new. I would soon come to realise how incredible this girl is. She genuinely cared about me, she made me feel secure, she encouraged me to be a better person. With her help I slowly started becoming a more independent adult. More confident, driven and self-assured. Life's never truly rosy but I was in a very good place with a far more healthy outlook on life.

Then I run into her completely unexpectedly. It was only a very brief exchange. She spoke twelve words. I spoke three. He who stood comfortably before thousands was suddenly filled with anxiety and fear. What was to her a meaningless exchange of pleasantries was the most intense moment of my already eventful month.

I don't hate her, how could I? Look at her, just look. She has goodness coming out of her ears. No, I hate what she turned me into. I hate the person I was up until the time I started forgetting about her. I haven't fallen back to that place, no, my foundations are stronger than that. But they were certainly rocked.

How does this insignificant human being have so much power over me?