You don't have to wait for pushing through an EHCP with the school, it's needs based not diagnosis based, and they legally have to action it as it is to determine whether the child needs additional support, not a guarantee of additional support.
Melatonin, if required, can be pushed through the GP, although it is very hard work.
DLA isn't diagnosis based either.
My daughters school, were reluctant to be named to take her as they do not have the resources despite our ECHP and funding awarded for additional support. They were very surprised we had managed to get this far without a diagnosis as they already have a child in her year who needs additional support that does not have an ECHP or funding currently, which they have to provide additional support for.
We have finally done the autism assessment and now awaiting everyone else to do their bit, before a decision is made. And we have a date for ADHD assessment which we've been told cannot be done before they are 6 years old.
Realistically the diagnosis wont change much, but alot of services such as respite is diagnosis based, and of course medication if ADHD is disagnosed.
Please push for the EHCP, and push with your GP for melatonin. You do not need diagnosis for either and the EHCP will secure the best suited school placement, and speech and language.
I get you though. My daughter self hurts in frustration, meltdowns that last several hours, selective mute at school, poor speech, poor sleep. It's tough, I wish you all the best.