poor thing =(
Personally, I would think that you would be doing her a mercy. You gave her a chance, and she just didn't pick it up. You could assist/force feed for the rest of her life, but it's gotta be super stressful. If she hasn't picked it up by now, I imagine that she isn't going to.
I applaud you for doing your best and sticking with it for over 2 years. I don't think anyone would 'blame' you, or say you did the wrong thing if you were to euthanize her. My personal opinion would be to go ahead and euthanize. Quality of life, no matter how 'primitive' BP's may be, just isn't there.
I wish you luck and/or condolences in whatever you decide.