Why am I flustered around the girl I am really into? How do I stop embarrassing myself? How do I stop overthinking?

I am handsome, I go to the gym, I am intelligent and I know that. Yet I am so insecure around a certain girl because I find her so beautiful in many ways and it's the first time I want to date to marry. I had more girlfriends than most people my age, but I didn't like their personality so much, so it was easy to get with them. I still approached this girl despite my nervousness, despite my impulse to run away. She probably saw I was quite flustered, and she was too, but she looks like that all the time.

Why am I flustered around the girl I am really into? How do I stop embarrassing myself? How do I stop overthinking?
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