How to stop yourself from chickening out, and changing your mind. i just want to do it!? so frustrated :(?

I would pick an exact day to do it ahead of time and plan accordingly. Plan what you're going to say, and when. Tell your friends you have to have an important talk with the teacher and ask them if they can wait outside the room, after or before class. And maybe you could even call your teacher ahead of time to ask if she can spare some time before or after class. If you have everything planned out ahead of time, then you will end up doing it. If this is really important, then you have to do it. I hope it turns out okay :)