Hello! I certainly suggest you do tell your teacher as soon as possible if you are not ready to do it. You can also tell her, if it's true, that you are getting help for the problem, but you just aren't ready as yet to take on this task. If you are getting therapy, what does your therapist say about it?

Yes, your teacher might be disappointed, but she can find someone else. In the meantime, please keep working on your issues, so you can one day take on such a job, since it would be quite an honor.