@Lavenderdog OK, first things first – deep breath! You’ve got this, it’s going to be OK. I was where you are about this time two years ago.
Don’t worry about the biting, it WILL end. They all do it and it is horrible – I think it was the thing I hated the most about having a puppy because she got totally manic and would attack me and I just didn’t know what to do, but it did end. What I learned is that if she’s biting you, she’s tired, especially if it's later in the day. Once she starts with the crazy eyes, she’s off to bed, and ideally before the crazy eyes start so you’ve pre-empted it. I taught a ‘go and lie down’ very early on to combat this manic biting nonsense because otherwise I was going to have a breakdown. When she started it, she got put in her bed and I’d walk away behind a gate/door. She would usually whine for a minute but then realise that being in bed was where she needed to be and she’d conk out.
Where she is sleeping (or not sleeping) during the day? I would be cautious of allowing her to sleep upstairs during the day if you don’t want that as your only option for life. Be careful of making a rod for your own back.
Remember the positives, as you’ve said – she’s now better in the car, which is excellent news!
I think this age is the peak regret moment, but push through – it will be worth it and you will get the dog you wanted. She’s in there somewhere.
Agree with @VanGoSunflowers – can you get just a bit of respite somehow? I was on my own Monday to Friday and made judicious use of friends with dogs in the village when I was just not coping. A break away from her made such a difference. I started leaving her quite early on for short periods because I had to do things outside the house without her. It depends dog to dog but for us there were no ill effects and she is a very settled, confident little dog now.