What you're experiencing is most likely fear of rejection.
It's like looking for a job. When you apply for a job there's always the possibility of being turned down but ask yourself, what is your chance of having a job if you never ask?
Body language is important. Don't be stiff and nervous like a formal presentation and a good sense of humor is a great help, just put a big smile on your face and say something like, "I think you and I would make a great couple, what do you think?"
If you happen to get rejected, ask yourself again, what would you do if you didn't get the first job you applied for? Remember, you will never gain at anything unless you are willing to take a risk.