What is a good way to admit your feeling for someone when anxious?

There is a girl I like and I know she is single. I'm extremely shy and we were talking today. I started to get sick to my stomach at the thought of asking her out and just didn't. By then end of it all I was shaking because I was so freaked out. At just thinking of asking her out. I don't know how to overcome this at all.
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It isn't rejection that freaks me out. I don't know why I react the way I do. It's like my body is rejecting what my brain wants to do. I signed up for therapy because this has been an issue as long as I can remember.
What is a good way to admit your feeling for someone when anxious?
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