What is the most concise way of expressing the sentence?
I am very very ashamed of my unacceptable and horrible behaviour.
I am extremely ashamed of my unacceptable behaviour.
I am extremely ashamed of my unacceptable behaviour.
There is no need to repeat the adverb ‘very’ twice. The adverb ‘extremely’ will suffice. The words ‘unacceptable’ and ‘horrible’ are similar. So retain only one of them.