only too well
only too (something)
Very. Don't tell her the truth—she'd be only too glad to use it against you later. As a small business owner, I know only too well how tough it is to succeed in this economy.
only too ˈwellused for emphasizing that you already know about somebody/something or have already experienced something unpleasant, etc: ‘Do you know Alex Humber?’ ‘Only too well. He’s not one of my favourite people, I’m afraid.’
See also: well