It was a mistake only if he was lying to his previous attorneys.
This likely explains why they stepped aside. He was misleading them too.
What may be the case ... Trump simply isn't smart enough to figure the difference between ethical and/or legal behaviour and unethical and/or illegal behaviour. He thinks whatever he does is just fine, so long as he gets the win. He is used to crossing the line and having the financial resources available to defeat any other parties to a contract with him.
But that isn't how the government works. Mueller has vast resources.
Trump's latest lawyers are developing a strategy assuming his behaviour was illegal, and so they want the horse put back in the stable. But I am afraid the horse is out. And Trump is heading for jail, unless he can get a pardon from Mike Pence.