It is not easy to cure shyness. Often, it's a lifelong condition that keeps you from getting as much from life as more outgoing individuals do. It's easier to get what you want out of life if you are daring. But it is true that many shy people grow less afraid with time.
You say that you are afraid of feeling rejected, but in fact it's your boyfriend who's feeling rejected because you're not showing enough interest and affection.
You can't go through life being afraid. You have to do things, even if they frighten you. It's like parachute jumping -- you feel great anxiety and don't really know where you are going to land. Did you make a fool of yourself, or will everything turn out okay?
Whenever you think positive and loving thoughts about your boyfriend, speak them out loud, without thinking first. Take the risk of being rejected instead of rejecting him. And I promise, you will not be rejected! People are happy to receive all the love they can get.