Children can be the most rewarding, and the most unnerving, things on the planet. You are definitely not alone. My girls are at about the same age spread as yours, only now they're 10 and almost 14. We had the same struggle with the hair issue, along with the usual list of other things. Children spend their days at the whim and mercy of other people's schedules. When they wake up, what they eat, what they wear, etc, etc, etc. They need to feel like they are in control. We need them to do certain things at certain times to function during the day. So we need to give them choices.
You want her to take care of her hair, or at least allow you to brush it. Let her choose the hair brush and the smell of her shampoo. If it's pink with polka dots and a squeaking Elmo, that's fine, lol - let her choose it. When you wash and condition it, run your fingers through it to get the snarls out - you might have to work at it a while. Use a prn med if you need to so you can get through it. While you don't want to brush the heck out of it when it is wet, I think a wide-tooth comb should be fine. If it's long, put it in a loose ponytail when she sleeps. She might not like the sound or the heat of the blow dryer. If you wash it at night and towel dry it well, it will be dry by the morning. Spray a little detangler or leave-in conditioner on it in the morning and give it a good brushing then (with the potential Elmo brush, of course!). Let her choose some barrettes or ponytail holders. Coordinate them with her clothes and treat them like special jewelry. Girls love to feel dressed up.
I know this might be very difficult for you. She won't do it perfectly. This is where your prn, if you have one, comes in. But if she feels like she has some control of the situation, she may struggle less and things will go more smoothly.
You can use this same strategy for choosing clothing, foods, bedtimes...it's a matter of being smart about the compromise. Yes, she needs to get dressed for school. That's not negotiable. What is negotiable is which dress she wants to wear. She has to eat a vegetable at dinner: does she want peas or carrots? Bedtime is at 8: does she want to read Snow White or Sesame Street? Try to limit the choices, or you'll be there all day, but giving her choices will give her control.
And make sure you're taking care of yourself too. Is there someone who could help with the bedtime routine? My husband works second shift, so it's usually been me trying to get dinner, homework, bedtime, etc all done by myself. It sucks some nights. It really does. It's not as bad as it used to be now that my kids are older, but my older daughter has Asperger Syndrome and can still be a challenge, especially around hygeine. Are you also trying to bathe the two year old? Can you rearrange when you do certain things? Somehow make dinner easier so you'll have more energy for bedtime? I know you said mornings were more difficult, but if you have the energy to get the tangles out at night, then the mornings will go better.
Sorry to ramble...just brainstorming out loud.
I also have bipolar disorder, with some anxiety and dissociation. There are some nights that I simply cannot cook and just getting through dinner is a challenge. I try to keep frozen, but healthy, food on hand that I can just put in the microwave. To get through bedtime on my bad days, I used to let the girls choose a short story with lots of pictures, so I knew I could get through the book. Some nights we just snuggled and watched Clifford or Caillou on PBS - something calm.
But giving them choices made a big difference, and understanding their triggers (like the hair dryer) helped me manage my own.
Hugs to you.