Ouch! Sorry about your predicament, I know it can be very frustrating!
We went through a similar ordeal when we adopted Thor. She was fairly friendly from the beginning, accepted pets and such, but had a terrible nipping habit. I really think she just picked up the bad behavior while in the shelter or at her previous owner’s. She would nip so hard that she would elicit loud yells, nip hard enough to leave giant bruises, and twice even broke skin! Even though she broke skin, I still classify these as nips, in my understanding when a rabbit truly bites they will dig in and hang on for dear life (terrifying bunnies!). Thor was only doing nips, but they were HARD and it got to the point where we were scared of her. It is difficult not to take it personally.
We tried a lot of things to make her stop: Yelping loudly or shaking a noise maker, then leaving after a nip so as to not reinforce bad behavior. I also started to pick up on her patterns, so when I knew she was coming in for a nip I would either leave or just gently pick up her front feet and turn her face away from me. This 180 turn confused her, and prevented the behavior. We also did briefly try the advice to gently push her shoulders down after a nip (some say this is the way mother rabbits assert dominance over kits): I wouldn’t recommend this. Even though it doesn’t hurt them, I think any physical consequences just don’t get through to a rabbit and it seemed to make her nipping worse while we were doing it.
Initially I despaired because our interventions seemed to have no effect. When we would yelp or make loud noises, she didn’t seem to react at all! Over time however, I realized that the amount of times between nips got longer and the nips that did occur got softer. (we started one of those “it has been __ days without an accident” counters on the fridge
) It takes time, but training like this does get through to them. It could also be that over time she grew to trust us more and realized she did not have to nip to get her way.
Good luck! Feel free to vent here, I’m sure it helps. Just know she doesn’t hate you. And that from my experience, even a serial nipper can become the sweetest bun <3 Its now been 51 days without a nip