The next time he says your marriage is over, calmly respond, 'You're right, it is.' Then immediately work out who is going to physically leave. That way, he has ended it, because he said it first at that point in time. Don't allow time for discussion for him to back-track and win you around again. Leave yourself, there and then, if you have to.
You currently have left. Hopefully you haven't gone back yet, and can stay gone.
Keep reminding yourself how awful he is, that it's a pattern that's not going to change, and how dreadfully unhappy he makes you feel.
Constantly suggesting breaking up, then pulling you back in, is designed to destabilise you. Those are not the actions of someone who loves you. It's control.
Every time you think about the good times, and what you'll miss, remind yourself about all the bad times, and what you'll gain by not being with him (stability, pleasure in life, healthy mental state, freedom to be you).
I know it's really hard to leave, I've been there. It's like an addiction - you always hope for more of the highs.
You can do it. Just take one day at a time.
Every day you spend without him, give yourself a pat on the back.
Every day you spend without him makes the next one easier.
Every day you spend without him puts him further into your past.
Every day you spend without him puts you further along your path to a happier life.
Good luck!