It sounds like your therapist is being honest with you. He believes your marriage is contributing to your condition and can't help you if you remain in the relationship. If there is no obvious reason that divorce would be in your best interests, I suggest you ask him or his supervisor (if he has one) flat out "Why?."

I had a therapist decline marriage counseling after meeting my then husband. The therapist told me that my husband had a type of personality disorder that rarely responded to therapy and he had never successfully treated anyone with it. He gave me options and offered referrals. I decided to see him solo. I saw him on and off for years and he was a wonderful therapist.

Good luck.