Sorrow. Accompanied by depression, fear, dark, cold... it was just another service the Dementors had to offer. Sorrow for every past sin you ever committed. Sad memories seemed to pop up twice as much since the happy ones took their leave. Bellatrix was sorry. Sorry she'd ever trusted her husband, sorry she'd agreed to go on her last mission with Crouch... sorry she'd failed the Dark Lord.
A long, long time ago, she used to feel sorrow for Rodolphus. She used to feel pity when Crouch was dead. Once, during a particularly bad night, she'd even felt bad for the Longbottom boy she'd spotted while torturing his parents to insanity.
Now she felt sorrow only toward the Dark Lord. Just like every other of her emotions. Sorry she had failed him, sorry she was of no use to him, rotting away in Azkaban... but when the glorious day came that her Dark Lord came to rescue them, she would redeem herself for it all. She would be more useful and more treasured than all of the other Death Eaters combined.
Maybe, somewhere, he feels sorry that she's still waiting.