If you have trouble composing yourself in person you might try distilling your thoughts in a note. That way you can sort out what you want to say without a live audience. Preparation can take alot of the anxiety out of that sort of thing. If you don’t trust yourself to express yourself in person but want to change that you should let her know in a less direct medium, like a letter or email. Chances are she’ll be understanding of your difficulty and having it out in the open could help both of you dispell it.