You may have to start your turnaround by forgiving yourself each time you bungle it. And we all bungle it at times.
In fact you can expect to mess up on occasion and either shut down or blow up. That's the nature of any major change. If so, forgive yourself.
At the same time, realize that when you leave a relational mess, you need to go clean it up. Apologize to the other person without blaming them for anything they may have said or done. This is about your business, not theirs.
The important point to know is that apologizing isn't to be used as a way to make your bad actions acceptable. They're not acceptable.
At the other end of the spectrum, if you continually shut down during a conversation, then you need to apologize for that, too, assuming the other person is a reasonable and rational human being. They don't deserve to be treated as if they're abusive if they're not.
Is there anyone you need to apologize to? Who? You may practice by writing it out if you need to. It starts with six words. I am sorry. I was wrong.