You do realize I hope that when BB is being so negative...that you can look at her and tell her that it wears you out to listen to her negativity all the time and well...simply, flat-out tell her....you choose not to listen to it at that time. Then let her own her own reaction to that. Remove yourself from the situation for awhile, go do something else.
Asking her not to be so negative isn't going to work with her obviously. However changing your reaction to her negativity...that you DO have control over, stop listening to it. Sure, if you tell her what I suggested it's going to piss her off....but you need to let her own her own reaction and emotions.
Is the statement true? Does it tire you to listen to constant negativity? Do you want to continue to listen to it? Do you want to continue to try to get her to see how things aren't the way she keeps portraying them (when that hasn't worked for you in the past)?
Well...if the statement is true "I don't wish to listen to your constant negativity, it wears me out." then say so....and leave her to deal with her own reactions and emotions from it. Don't tell her how to fix it so she doesn't come across that way, she needs to figure that out (which BTW she won't do, as long as she has you for her sounding board.)
I'm sure she's even going to get you to buy that you are invalidating her opinion...don't pick up that rope. You aren't invalidating anything, what you are refusing to do is participate in something that is exhausting for you. It has nothing to do with one's opinion. If she feels you are invalidating her opinion....let her deal with that, don't pick up that rope. Validate her in other ways throughout the day....like for example, when I was telling you that you told her something she did was "Ok", but not that you were "Proud" or whatever that was (sorry I forgot). If you will validate her, without holding back, and show her you are proud of her achievements...or proud of the woman she is (in whatever way)....and refuse to pick up the negativity rope, you will see behavioral changes. They won't be instant, but they will happen.
Just my 2-cents. Honestly Lou, it's that kind of honesty and owning my own reactions/behaviors....and NOT rescuing my H from his reactions/behaviors that has helped us the most.