NIce write up man. it takes balls to write something like that. I hope that we can help somehow.
I feel like this is basically what it comes down to. You recognize the problem. You recognize your introversion. Now, recognize that you've got nothing to lose, so just go for it. Like others have said, say yes to your friends when they ask you out. You don't have to drink, be the DD. Or, just have one drink and make it last all night. Maybe it'll suck, maybe it'll be fun, but you'll never know unless you get out there.