I would write a letter, get it all written down, the reasons, your defence of the situation including all the relevant details, the whys.
Also the details of why it was never disclosed, how you felt ashamed, how you feared the truth escaping (with and without explanation.)
The years of blackmail you have been subjected to and how that has affected your confidence.
Just get it down, every angle, everything.
Read it, read it again and then edit to what you feel comfortable.
Then leave it.
Then leave him, if that is what you want, then see if he reveals all.
Then you will have this letter to either give them or quote from.
Do not let this man steal your life by blackmail, it is an imprisonable offence.
What he has done is worse.
The operative word here is THEN.
Don't reveal anything until you must or it is a time when you feel comfortable.