i have been where you are, my old english, developed hip displasia
he was 13 if i remember correctly, he cold no longer lift his hind legs up he could no longer stand at all, i tried all the medicines available at the time, the cost did not matter, i just didnt want to lose him..
unable to toilet himself and the look in his eyes, i knew it was time
to let him go over the rainbow, heart wrenching... still makes me wellup even now.. just be with her at the vets and talk to her, its quick and painless...then i suggest you go have a stiff drink, and be with someone.