I need insight on my toddler

Hello I would like to ask in a post on some insight with my toddler??

My son will be 3 April 30th
He get very frustrated easily. He is speech delayed and I have him in speech therapy for it. The problem is when he has melt downs he bang his head against anything near him, if we’re outside he throws himself backwards on the cement and hit his head. Most of the time I’m there to prevent him from hurting himself other times I just let him be if he isn’t going to truly injure himself.
I talk to him to try to get him calm and try to redirect. Sometimes he listens and sometimes he doesn’t, like I’m not sure if it’s him understanding trying to get his way not matter what or he really doesn’t understand. I worry for him and I’m so exhausted.

Thank you all