Blankets and pillows! Our son NEEDS a safe sensory corner in several places around the house. I don't even talk to him when he's in that "state." We just guide him (or carry him) to the safe corner and he burrows under the blankets and pillows. If he kicks, hits, bites or head butts, he can't hurt himself or anyone else and all that sensory input is just fantastic to get him calmed down. It took us about a month to show him to do that, now I sometimes can just say "blanket" or "pillow" and he'll go right to his spot and do what he needs to do.
My son won't go near anything with Omega3's. We have tried EVERYTHING and he can smell it from a mile away. I wish his sense of smell wasn't so darn keen! Hopefully someday it will save someone from a fire or something...
In the car it's harder. We do keep a weighted pillow for my son, but sometimes distraction works best... My son has gotten so bad in the car I've had to pull over. You MUST do this if your child is taking most of your attention or if you are very upset. Sometimes stopping the car makes them react. Don't interact while you're stopped. Offer distractions, like: "look at that bird" or "did you see that truck?" and offer a pillow or blanket if you happen to have one in the car. I also ALWAYS carry snacks and juiceboxes with me. Sometimes my son is hungry or thirsty and isn't able to tell me. I try to offer those as well. Last resort, get yourself calm enough to be able to drive and get to your destination as soon as you can.
It is much easier to deal with the meltdowns at home. The car and the community are tough. I'm glad you are getting help. It's hard when you first get the diagnosis. With time you will begin to see patterns or triggers that will help you anticipate when these things are coming. You will also learn "tricks" to divert the situation. Good luck! You are in my prayers!!!

Sensory Overload, like too loud of sound at the theater, can be adjusted, as there are now theaters in Spokane that now limit the volume for Spectrumiters. Even still, as a Grandpa, I will thrash around during sleep UNTIL the Heavy Blanket becomes my neurological focus.
A Bean Bag, or popcorn in a sock, resting in my hands or lap, was enough to shift from Visual Overload to Touch Balancing. Other Stimming activities, like spinning in place, hand clapping, twirling hair around the finger, RUBBING a Worry Stone, or finger snapping are other examples of mitigating meltdowns. I pace to let off Social Anxiety or bicycle 40 minutes REAL HARD to wear out the ebby gebbies in my muscles. Hey, it's not drugs, or food for obesity.
Still, I can get obsessed counting signs down the road at the stop light, and DIVERT The Focus by whistling (visual overload which I cannot control, to something i can control, Sound Of Whistling). (125)

My daughter does best when she is left "alone", as in, we are with her but don't touch her or talk to her. It also helps to eliminate as many triggers as you can, and it takes time to figure those out. Every child is different and I think you may have to learn by trial and error to some extent what works for your daughter. One thing that I think helps almost universally is to minimize your child's exposure to sensory overload (loud sounds, lots of people or things going on, "busy" things to look at) and especially in a meltdown, or when you see one coming on.

Listening to classical music. It just does something to their brain that is very calming.

They don't mean to... listen, I was the "expert" offering my advice to parents of kids with ASD for over 15 years and then I had Nathan. Even with all my book knowledge, all my "expert" colleagues and friends, it is the experience and advice from parents on sites like this one that I value the most because we LIVE this life! Hope things keep getting better! Hang in there and remember it's one step back and two steps forward on this journey! Don't be discouraged!!! Always come back for more support from everyone here. Blessings!