Somebody locked this ahead of me but apparently didn't write anything. I too was too introvertedin high school to approach any girl I was thinking about. In college it was easier--but it really does take time for introverts to learn how to approach a girl--since the more you feel excited about a girl, the shyer you will also feel. It's a normal emotional effect, and it's one reason why so many young people use alcohol to disable their shyness-brakes so they can say something without overthinking it first.
I'd say your present situation isn't very good for being able to start something up, since the girl is apparently attractive and (perhaps more outgoingt/extraverted than you) to easily get herself a boyfriend whenever she's ready for one. If it's any consolation to you, there's good research that has found that relationships between an introverted boy and an extraverted girl are more difficult to make last than when when the boy is more outgoing than the girl. On the other side, introverted guys may get a later start with relationships than those that are more extraverted or more practiced at parties in approaching women with their shyness-brakes dissolved in alcohol.
One more issue is important here: It's a lot easier to keep a fantasy love affair going on in your mind (with the idealization of the woman that ALWAYS lasts until you've gotten to know each other for several months) than it is to get gradually close to a girl that doesn't have the years of "perfect shine" you've built up for the one that got away.
But I do have a suggestion for a "personal training project" you can engage in that would get you ready to start engaging women and getting close to them WITHOUT PUTTING THEM ON A PEDESTAL FIRST like a PRINCESS. 1. Pretend you are a romance novel writer. Start writing opening scenes for a couple that meets in some quirky, unusual, or quite normal way--like meeting in an aisle at Walmart, spilling a drink on her at a baeball game, meeting a girl in an online chat group, meeting at a "meetup group" for an interest or political cause. Write scene after scene--with some of the guys being pretty similar to yourself (as you think of yourself now) but others different.
2. A really good trick to use in a story, and then also to try out in a real situation is for the guy to follow his INTEREST or his HUMOR/FUNNYBONE instead of his erotic turnon--because if you just make a comment about something that's funny to you, or interesting to you in a situation (or ask a question about a kitchen product at Walmart) you won't get blocked by your snyness-braking system because you're not focused internally on your feeling turned on or blinded by her beauty. Relationships for women are more likely to begin with enjoyment and comfortable feelings than with erotic excitement (unless it's night and party-time with alcohol to set up the mating mood.