You know what I found to be really successful. When my toddler started having tantrums I learned this method where you acknowledge their feelings by saying “I know your upset” then saying “it’s hard not getting what you want” something like that then saying nothing else and being firm with the boundary you set. Also not just saying no, explain why they can’t do what you’re preventing them from doing like “you can’t watch tv because you have watched enough tv today”. My toddler doesn’t have tantrums as much anymore. And if you have to leave the situation to stay calm just tell them that you are going to do laundry, go to the bathroom whatever so they don’t feel like you’re punishing them by leaving.
My daughter turned 3 this summer and that's how she's getting. I usually acknowledge her by telling her I'm sorry you're upset by this, and explain like hey we can do this tomorrow or something. A big thing is what causes the tantrum and try to work with his feelings. He doesn't understand why he feels that way or may not even understand what it is he is feeling. Just keep that in mind
Toddlers can’t process information while having a tantrum, wait for the tantrum to be calmed down before talking about solutions
That’s why I acknowledge how he feels, “I know you’re sad” “I understand that this makes you feel upset” then saying “It is upsetting when you can’t do or have what you want” (this part sounds strange or mean but that’s how they connect how they are feeling to the boundary you made) then they can see that they are upset because they aren’t getting what they want. This helps them gain self awareness and emotional regulation. Then don’t say anything, nothing just stay silent until they calm down. You can even comfort them but also ensure they aren’t doing anything dangerous. It’s also important to stay calm, being regulated helps them regulate.
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My daughter is on the spectrum, although not officially diagnosed, and the tantrums are so hard. Look into CFT practice for therapy it helps us a lot. And also change your perspective and work on your own emotional responses. Your child depends on you to be regulated and learn from you how to regulate their emotions eventually. This is the hardest work imo, and absolutely crucial, and it will take long time so remember to be gentle with yourself. Find yourself grounded no matter what emotion your child is experiencing and how hard he’s screaming. Let go of the need to fix it for him, just be and in time it’ll get easier