You just described every guy in highschool. You're not a unique snowflake.
I'm going to tell you EXACTLY what your problem is, but like every other guy on here you're going to deny or ignore this advice.
Your problem is your ego.
You're like a fat guy who's over weight but who can't seem to understand that his over eating is causing his weight gain.
Being shy, or nervous about rejection, comes from your ego. Don't believe me? Read anything by Eckhart Tolle ( link )
When you are able to let go of your ego, you stop worrying about rejection simply because you're not letting others dictate your value. As you mature and experience the world enough you'll start dictating your own value, instead of upon the approval of others.
Many guys base their self worth entirely upon the approval of others. Super models suffer from this because their identity is based upon the perceptions of others. Body builders have this same problem. They work out desperately needing to be judged against others.
The deep fear is that you're not good enough.
But as you learn to build your own self esteem you'll stop needing the approval of others, and therefore you will no longer need her to say "yes" to your date proposal. Instead you'll be more focused on having fun with her, based on your curiosity of who she IS, instead of focused on "am I good enough for her."
Remember, you're a great guy who's looking to meet a girl who's good enough to keep your attention... NOT the other way around. You already know how great you'll treat your woman, and now you need to meet one worthy of your time.
My Blog ( link )
P.S. A great way to practice your social muscles is to force yourself to chat with every person you meet through out your day. Your job isn't to make these people like you, or to get phone numbers. It's simply to practice and learn that art of small talk.
Chat with the fat girl in line with you at the grocery store. Chat with your hot waitress. Chat with the old guy who's sitting next to you in the mall food court.
Learn to chat with EVERYONE and you will have NO PROBLEM chatting up the hot chick you just met on the bus.