We have not had RSV, but we have a little guy that has needed neb treatments often (every cold he gets goes to his chest, and makes it hard to breathe). I cannot remember the ages that we given to me for this description but it was something to the effect of:
from birth to about 12(?) months, they do not care about the nebs.
From 12(?) months - 18(?) months, they may fight you about getting the nebs.
Starting at 18(?) months they will go get the neb when they need it.
Keep doing the nebs - even if it hard on you and her. If she cries while you are doing it, at least you know that it is getting deep in her lungs. :)
I sound heartless with the above comment - but I have sometimes had to pin my little guy down to do them. It breaks my heart, but I also know that he NEEDS to breathe better, and that this will help him get there. It does not make it fun for either of you.
I agree with the other comments about not needing the strap around the head. We got rid of that a long time ago.
For us, if you can talk her into it, it has helped to let him hold it. (He sits on my lap, and I rock him while we do it, but it gives him a sense of control with it. He is older than your little person, so this may not work for you, but for us it helped.)
Good luck and I hope she is feeling better really soon!