My heart really goes out to you (((((((Anjelmarie))))))

Does your BF know how badly this is effecting you? Can you sit down with him and have a heart to heart conversation? If he insists that his niece must continue to stay with you for the time being, can you at least try and work out a set of "House Rules" with your BF, which you can then give to his neice (rules like: No knocking on your bedroom door if it's closed etc). I think it's also totally fair enough to tell your BF's niece that you need some "quiet time" each day. That is completely understandable and I'm sure that wouldn't hurt anyone's feelings.

I am so very sorry you are in this stressful situation when you're already dealing with the stress of your depression and anxiety