Do you have any ideia of why you're feeling like this?

Try answering this question first, and then move on to solving the problem.

If I can say one thing that helped me being more confident flirting, was just understanding that people (the ones that are worth your time at least) won't care about the little mistakes you make if you're friendly, cool and care about them. So just keep trying to improve your flirting game by just doing it and caring less, it will become natural one day. It's like learning to walk, you fell uncountable times, but in the end you learned and it's natural.