Very bad behaviour

These few weeks Hong Zheng seem to able to ulter a full sentence. For example, "Hong Zheng goes downstair." He seem to able to catch up the sentences ulter by us at anytime. However one thing really worry me is i started to know what he has learn and that really put me in a very nervous situation. He likes to say "go away". He said "Hong Zheng goes away", "Mommy, go away" and "Daddy, go away". When he was not agree to something that i asked or he was losing patient on certain things, the first words will be " dont want lah, go away". He hit his head occasionally too. All the words and action that i did half year ago he learned up.

I was really very upset. I was so regret on what i did. What can i do now? Anyone can help me?