When you're young, everything seems much more dramatic that it will be when you become an adult. The fact is, very few people live their lives with the first person they had a mad attraction for. The key is to understand that and consider your life right now as a learning experience, not the final product.
There are risks to everything you do in life. When you apply for a job, you may be turned down. When you take a test, you may fail and when you ask a girl for a date there's always the chance she'll say no. So who are the people who succeed? They're the people who are willing to take that risk and try. The only way to be sure you will fail is to do what you're doing now.
Look at this as a training exercise. Practice asking other girls until you get some experience at being turned down. Once that happens a few times it isn't so bad.
What you're going through is what everybody else that's ever lived went through. Don't take it so seriously. Learning how to deal with girls can be fun but like playing a sport, it isn't fun until you practice enough to get good at it. Relax and have some fun.