Hi, I'd like to help you with your question.
It sounds like the main issue here is your husband's insistence that his ex wife still be part of his life. He is not putting you or your feelings first in this situation. Instead, he is putting his ex's dependency and his own feelings before yours.
What he should be doing instead is listening to you and finding ways to reduce the contact he has with his ex. Since his children are grown and out of the house, or close to it, then contact with his ex, except for special family events such as weddings and graduations, should be done. And when he does have contact with his ex, you should be there or somehow involved.
It would help greatly if you could go to therapy together to work on this. If his ex being on your insurance is preventing you from attending therapy, then try other alternatives. The local community mental health center has therapists that can help. Or you can find a Master's Level therapist that will see you for a reduced rate, such as $25 or 50 dollars an hour (some therapists will do this). Or if you attend church, your pastor and their staff are often very good with marital problems.
Tell your husband that you are no longer willing to accept that his ex wife is still on his insurance. Let him know that you also will not tolerate any other contact except when absolutely necessary. When you tell him this, be sure you define when you are ok with seeing his ex and when you are not.
If he will not cooperate, then let him know you consider this a danger to your marriage. It is, in essence, a type of affair because your husband is emotionally involved with another woman. In any marriage, it is supposed to be two people putting each other first. Your marriage now has three people in it.
Decide how you want to respond to his refusal. Is it worth ending the marriage? Do you feel a separation would help?
Before you decide, consider doing some research to help you make an informed choice. Here are some resources to help you:
Ex-Wives and Ex-Lives: Survival Guide for the Next Wife by Paula J Egner
Stepcoupling: Creating and Sustaining a Strong Marriage in Today's Blended Family by Susan Wisdom and XXXXX XXXXX
Taking Space: How to Use Separation to Explore the Future of Your Relationship by Robert Buchicchio
Should I Stay Or Go? : How Controlled Separation Can Save Your Marriage by Lee Raffel
You can find these books on Amazon.com or your local library may have them for you.
The main point here is for you to have some control over the situation. Letting your husband decide and resist changing the situation has left you with little choice in the matter. You should have at least half the input about what happens in your marriage. Taking steps to change that will help you feel better and decide how to best handle the situation.
I hope this helps you,