OP, on the subject of biting your nails, I am in my 60's and apart from a very short spell when I was younger, I have ALWAYS bitten my nails. Then, I was on MN one night, and saw a post by someone asking for help to stop biting her nails. She had posted a picture, and quite honestly her nails looked really sore, and quite revolting, just like my own. She was desperate to stop, and like you and ME, had tried EVERYTHING. There was the usual advice given, but people like you and I know that even these things that taste disgusting really don't work, as you get used to the taste, no matter how bad it is. Then someone suggested 'tapping'. I'd never heard of it as a cure for nail biting, and wondered how the hell something like that could stop you biting your nails, I even thought maybe they'd responded to the wrong post, but the reader included a link about it, and as I too was desperate to stop biting, I decided to give it a try. I'm afraid I can't find the post or the same video now, but if you Google 'tapping to stop nail biting', I'm sure you'll find lots of videos on YouTube about it. Basically the idea is that you replace the habit of nail biting with tapping. I know, it sounds ridiculous, doesn't it? But to my absolute amazement, it worked!! Initially, I just started tapping my chest every time I noticed myself biting, not quite like the video showed, but my own version of it, and over a period of weeks, my nails actually started to grow. There were no whites for a long time, as I'd bitten them so badly that I was more or less biting my fingers as there was not much left of the nail to bite. I armed myself with several glass nail files, and put one in every room, plus one in my car, and one in my bag, and every single time I felt a slight catch where my nails were starting to grow, and where previously I would have bitten them, I would file the catchy bit off, until it was smooth again. It's taken probably about 3 months, but I actually painted my nails for the second time tonight, and not just with a clear polish, but a colour that actually shows up. I am, as you can imagine, extremely proud of myself. I'd highly recommend giving it a try, after all, what have you got to lose, it doesn't cost a penny, unlike all the other methods you and I have tried over the years, and if it worked for me, it just might work for you. If so, I'd love you to DM me and let me know. Good luck.
Oh, as to the other thing, I too strongly recommend ditching this guy, and going back to live with your parents, assuming they live close enough to your current working life to make this feasible.