Is it possible to get out of the arrangement you are in with him? If so, that should be the first step you take. I know it's difficult to do this in a business arrangement, but I thought I would put it out there in case there was a chance you could do it.
The next thing you could try is talking with him about the boundaries of your relationship. Tell him that talking about you with others in a negative light is slanderous and hurts you and your business. You may also want to talk with your lawyer about action you could take if he continues. This is a bit extreme right now, but it never hurts to be prepared. Relationships that end badly sometimes get out of hand and you want to protect yourself.
Any person that talks about you behind your back but is nice to your face is not a friend. He is hurting you and he knows it. It may be his way to get back at you for how he feels and in that case, he may not stop if he feels hurt enough.
Is there any way you can divide the responsibilities of the job to lessen the chance of you running into each other? If you can, try avoiding him.
Can you bring on other partners and lessen your need to be involved? It may not be great to bring someone into a situation like this, but it may help you in the long run if you have others that see his behavior.
You may also want to get lots of support with this problem. The more people you can turn to to express how you feel, the more chance you have to vent and feel better. Consider seeing a therapist. Talk to your doctor for a referral. You can also search on line at www.psychologytoday.com.
If you have supportive family and friends spend more time with them doing something you enjoy. Join groups that interest you such as hobby or activity/exercise groups. Take time to pamper yourself. Get a new haircut, buy a new outfit, travel for a bit. Anything that can take your focus off this toxic relationship and lets you get a new perspective.
I hope this helps you,