Hello, I'd like to help you with your question.
It sounds like your husband's paranoia might have already been there before and the relationship between you and your ex might have triggered it. That is not to say it is your fault for your husband's reaction. It is very normal for a spouse to be extremely upset at finding out about an emotional relationship like this, but accusing you of things that are not even related to what you did
is hurtful to you and unfair. His accusations are more about his own possible illness than about what you did.
If your husband does have paranoid personality disorder then he most likely has taken this incident and has run with it. He already has a difficult time trusting due to the possible illness and gaining back his trust will be even more difficult for him now that he is convinced that you are doing things to him or behind his back.
However, if you feel you still want to work out your marriage, then it is worth trying to get him to work with you. If he will not listen to your attempts to talk to him, then you may have to ask others such as friends or family he feels he can trust to intervene for you. If they can approach him and let him know that they support the two of you being together, he may be able to listen.
You can also ask your husband what he needs from you in order to rebuild the trust between you. By asking him, you can get specifically what he is looking for in order to reconcile. While this may not work as the lie detector did not work, it is worth a try.
It may also help to write him an email or letter telling him steps you are taking in order to fix this issue. For example, let him know that you are no longer in contact with your ex. Tell him that you are willing to go to counseling with him or on your own. Tell him how you feel about him. Things like that, in an email or letter form he can read over and over, might help.
If all your attempts don't help, you may want to talk to a counselor on your own. Being in a relationship with someone who may have a personality disorder is difficult and draining. They do not react in the same ways to problems that someone without a disorder might. So you have to find the right way to address the issue that doesn't make it worse and the effort involved can be taxing. By talking to a counselor, you can get support for yourself and you can talk about the situation and get feedback so you know if you are on the right path.
Also, consider learning more about personality disorders. The more you know, the more tools you have in order to deal with your husband's reactions to you. Here are some resources to help:
I hope this has helped you,