Two stand out for me.
In second year of college, I lived in shared accommodation. A new girl joined and I got to know her a bit. We got on very well and I ended up falling for her. She was smart, fashionable, cynical, good fun, well read and pretty. She was so impressive and I though the world of her. At the very end of the year I told her how I felt. She listened but explained that she didn't have feelings for me. I sent her a text after we met but she never responded. I think she was trying to do me a favor by breaking off all contact but it hurt a lot. It really distracted me from my studies and I failed one of my exams. Then I ended up working in a crappy summer job just so I could stay in the same city we studied in. I was miserable and very lonely. It destroyed what little confidence I had and it took a long time for me to approach another girl again.
Fast forward two years later to my last year in college. I meet a girl who's the polar opposite of girl 1. She's really earnest and uncool, she's not fashionable, very eager. She was a bit crazy in a sweet kind of way. She ended up having a successful career in charities, which kinda tells you a lot about her. And I feel for her too. I was so scared of ruining my finals that I never did anything about it.
I found out afterwards from a mutual friend that she wondered why I never asked her out. I still think of her to this day.
It's very hard to know what to do when you have a crush on someone. These things aren't easy anyway but for shy people I think they are a lot worse.
They offer us so much that we crave more than everyone else, validation, companionship, acceptance, understanding etc. And we need that stuff more than other people. And people call us needy for that, and think that we are weak.