Love & Relationship Advice

I have been in a relationship with a man for 3 1/2 years. At the same time he started dating me, he also started dating another woman, and over the years he has continued to have a relationship with the two of us. He says he loves us both and doesn’t want to give up either of us. We are all divorced after long marriages, and are intelligent, educated and successful professionals. We all hate the situation, including him! The other woman and I share equal time as if it was a custody arrangement, and have NEVER seen each other at the same time and never will. Neither of us dates anyone else. This is making all of us depressed, angry and sad, but we don’t seem to be able to stop. Have you had experience with any similar relationships? Do you have any advice?