Issues with 4 year old

I have a 4 yr old girl who was adopted at birth. She tested positive for lots of goodies at birth and had to be detoxed. Most of her development has been on target but we did have an OT when she was an infant for some hypertonia in the arms which cleared up. Then a speech therapist for about 9 months when she was 2, but those issues cleared up as well. DD is smart, artistic (her preschool teacher is amazed at her abilities in the arts and with rhythm), funny, curious, charming, sweet, and loves people. She also has the energy of 5 children, almost no attention span, is aggressive (horrible biting phase from 2-3), and lives to fight me on Every. Little. Thing. My gut says there is some ADHD going on, because, well, I have ADHD and recognize it. I was given a referral to get her evaluated, but the had such a back log of appointments that they told me to keep calling back for a cancellation. Um, I have ADHD too, and have not followed through with it. But now a fire is lit, and I am going to make that happen. I have become the parent I swore I never would become. I yell a good portion of the day. I resorted to spanking. I threaten a spanking. Things I deplore in others, I have found myself doing and I go to bed every night ashamed and sickened. We have only the one child, because after a short stint of fostering a newborn and taking care of her I was at a breaking point emotionally and physically. DD is in preschool 2 days a week for 3 hours each morning, her teacher says shes fantastic, but has a hard time after snack and gets grabby. Teacher became very condescending when I mentioned getting her checked out for ADHD and has passive-aggressively refused to fill out the paperwork I gave her. Which makes me second guess myself and think I am the one with the problem, and DD just drew the short stick for a mother. I cannot tell DD no without a dramatic fight. We also have pets that she loves, but hurts. We have a cat she drags around like a stuffed animal, and the cat is VERY patient. I saw DD trying to pull kitty up on the sofa by a paw, kitty yowling, DD ignoring the yowls and only let go when I screamed at her. Oh, and my VERY loud child is also noise sensitive, so she freaks out with loud noises, like my screaming at her to let go of the cat. I feel like I am failing this beautiful girl of mine. It doesn't help that I have a highly critical, un-helpful family that I now avoid. If I have anything to offer prospective adoptive parents it is this: NEVER, EVER tell your family the drug history of your child's first parents. No matter how supportive they are in the beginning, it can change in an instant. Sigh. Sorry for the book, but I could use to some advice and some commiseration.