Be patient and take a hard line. Make a real finite boundary and enforce it. Change your email. I blocked him from mine. I got a new phone number (sure it's unfair, I had the same number forever, but it's worth having him out of my life). Go cutting the strings every place he's attached to you, e.g., financially, socially.

Avoid him. Go to new places, meet new people, do new things. It's not fair. It's not fun that they get to "keep" some of the friends or family after you go NC. But it's so much better than staying in contact. It takes time to build a new social structure, but little by little you can. Even work-wise, you can gravitate further away from him -- and the nice thing about professional relationships is that there is a real obvious code of conduct that states "this is appropriate" and "this is totally inappropriate."

Full, true NC is a gift you give yourself. Other people may not "get it" or it may be hard at first, but it's totally a loving gift.