I need help.

I'm seventeen years old and I go so a small private school in the US. While I've never been diagnosed with social anxiety, I do believe I have begun suffer from it to a small extent. I have no trouble conversing with strangers who strike up conversations with me, but I've never been one to strike up conversations with anyone other than my close friends and family. I'm an introvert to the extreme.

I wasn't always this way

I used to be alpha as fuck. A couple years ago I had somehow worked my way up the social ladder to become one of the most popular kids in my small private school. I was being invited parties left and right, I was known by every kid in my school, life was good. I had even become relatively popular with the ladies, seriously dating a very attractive girl [8/10] and getting very close to a couple other girls, [7/10] and [9/10].

But after a short amount of time and a series of events that I'm still not entirely aware of I'm nearing the end of my junior year a completely different person. I'm an extreme introvert only ever talking to a few close friends, and without any series female interaction, outside of casual conversation, in 2 years.'

My recent failed attempts

I only became aware of this subreddit a few days ago, I immediately read all the sidebar material but unfortunately I haven't put it to good use.
At a convenience store when checking out I attempted to have a conversation with the female cashier who I was not attracted to, just trying to make conversation.

"How are you?" -quietly and quickly

"good." -quietly and quickly

"have a good day" quietly and quickly

something so quiet I couldn't hear.

I had a brief boring conversation with that [7/10] I mentioned earlier. Every other time I've had the opportunity to converse with a stranger I never even make an attempt.

How and where can I meet/pick-up new girls?

I need your guys' help. Firstly, I'm 17 so no bars and preferably I'd like to meet girls my age. Secondly, since my high school is so small, of the girls there, there are few attractive girls and none that are either single or I that I haven't already been friendzoned (see the [7/10] and [9/10]). There aren't too many social events or opportunities so I'm pretty much stuck with finding girls outside of school. This afternoon I went to the library to find a girl to strike up a conversation with, I saw two girls sitting at a table talking but of course I got the bitch butterflies and didn't even get close enough to see their faces. Then I went to Book-A-Million with no luck again (although as I was leaving I did smile at a girl with her mom and she smiled back). Granted it is a weekday but still there have to be better places than bookstores where I can meet girls.

I want this

Though it may not seem like it, I do want this, and I hope that putting my plans in writing will help to motivate me. I am fully aware that this is all in my head, while I may not be supremely attractive my moderate past success demonstrates that my looks aren't a major issue. I tell myself over and over that the worst outcome is that their girl ignores me but for some reason I still can't do it. I have made other efforts to improve my life, /r/NoFap , /r/100pushups , /r/coldshowers (on occasion). As many of my close friends are finally getting girlfriends, I know it can't be as hard as I'm making it seem. Please, any and all advice as to how and where can I meet/pick-up new girls would be greatly appreciated.