You need to realize that this girl isn't a snowflake. She isn't unique. It sounds like you've really built her up in your mind and she probably won't meet the expectations you've unintentionally set. You've known this girl for years. Why didn't you ask her out right away? There's a big chance you are now in the dreaded "friend zone". This isn't a romantic comedy. You can't play coy with a girl and then make your sexual intentions known later. If you have the slightest inkling that you may like a girl, you should make those intentions clear from the start. That way you don't build her up in your mind and get nervous when you ask her out. Or worse you get depressed when she doesn't want to go on a date with you.

You should never put all your eggs in one basket. You should always be talking to multiple girls when you are looking to date. This way if one girl doesn't feel the same way about you, you have other options. Options are important. It's unhealthy to lust after one girl. (Obviously I'm not talking in the context of marriage/etc.)

Whatever you do, do not tell her outright that you have feelings for her. Even if she does like you, this will still put her in a rather awkward position. My personal opinion would be that you should look for another girl and forget about this one, but I know from personal experience that those words aren't easy to hear and you won't listen to that advice.

So, here's what I suggest you do. Ask her out to something simple like coffee. It sucks to say, but if you guys already go out together to places like the coffee shop and etc. you will probably have to spell out that this is explicitly a date. This is another reason why you don't want to wait on something like this. You need to either ask her in person or call her. Do not text, Facebook message, etc. Girls like when they can see/hear you ask them out. They size up your body language so make sure that you stand up straight. Say something like, "Hey, we should go out for coffee this Saturday." or "I'm going to the batting cages Friday, you should come with me." If you guys do a lot of hanging out already (and it sounds like you do) I would be more upfront. Something like "Let's go out this Friday. I'll show you the best coffee place in town." (I think you get the point.) If she declines your offer, try not get hung up on it. You probably will because like I said you've built her up in your mind, nevertheless, do not dwell on it too long. There are plenty of girls out there for you that are just like the girl you are lusting after now. Don't forget that you should be the prize, not her. She is the lucky one, not you.

It sounds like you really have high hopes for the future. Even if you two do go out, you probably won't end up getting married. That's OK, though. Like I said, she's not a snow flake. Nobody is.

"There are lots of guys trying to win her but she hasn't fallen for any of them...yet."

No offense dude, but if the girl is half as beautiful as you say she is, she is most likely having sex and has dated people in the last few years. I hope you have too. Don't get hung up on one girl. Anyway, I hope none of this comes off too harsh. I've learned a lot about girls the last few years of my life and the biggest thing you need to realize is a girl is just a supplement to a great life style. Don't base your happiness off of them. Do not build them up. They are human beings who eat, sleep, and shit, too.